Thanks for the tips eveyone. Approximate cut, then cover with fabric is probably the way I'd go.

Should just be a matter of collection & delivery now. All being well #1642 may be in its new home around the end of this coming week?

with a good wind behind it I think that's likely.
so pleased it's going to a good home.

I'm not sure how much this will help you in the UK, David, but I've been researching this, as I was thinking of making a hard case for my Railboard when it comes in. Maybe the info below can help you to find something suitable on your side of the pond.

What I think you want is polyethylene foam. You may have seen this in some packaging. It looks a bit like styrofoam, but is very flexible and very tough. Unfortunately it's expensive. I've found some sellers that offer sheets suitable for Sticks that are made for long-gun cases (shotguns or hunting rifles). The ones I've found run about $65 US for a hunk big enough for a Stick case, 4" thick:

https://www.amazon.com/dp/B015JRXH1W/ref=cm_sw_em_r_mt_dp_AX439QR8CZJW2JPJR564?_encoding=UTF8&psc=1

However, I note that many cases (maybe even the hardshell cases from SE) do not completely enclose the instrument, instead having a block at each end with a recess for the headstock and the pickup block, and a narrow third block in the middle. No need to have to fill the entire case, and leaving some empty spaces allows for stowage of extra pieces of kit. That said, I agree that covering even polyethylene foam with fabric is a nice touch.

It doesn't save a ton of money, but here is a listing for smaller chunks of polyethylene foam. Note the picture at the left showing the spring clamp and how rigid this foam is. This particular listing includes 10" x 8" x 2" blocks for $13 US each, so four of them (which would be needed for the case I have here, 7" wide inside and a 4" depth) would come to about $52. If I cut them to provide 10" of support at each end and cutting them down to the 7" width would give me 4 pieces 1" x 10" x 2" which could be glued together to form a center support. I could also get away with using the 8" length which would fully support the headstock and butt end, though not entirely under the pickup block, but would also give me 4 pieces 8" x 3" x 2" high which would give me a more robust center support.

https://www.amazon.com/dp/B08ZJYWNBQ/ref=cm_sw_em_r_mt_dp_RBASDHXMS02ZXJ37WRBS?_encoding=UTF8&psc=1

One last thing. Cutting the outline isn't hard, especially with this very firm foam. It cuts like a dream. One of those thin utility knives with the snap-off scores, fully extended, using a straightedge, will make the vertical cuts to accommodate the instrument very neatly. The problem is getting the bottom of the cutout flat. A company called FastCap sells this foam as "Kaizen foam", and it's a bit cheaper. A 2' x 4' x 57mm (2 1/4") sheet is only $55 US per sheet. They also sell tools for cutting and shaping it, including a scraper for leveling the bottom of the cutout. Although in a pinch, I wouldn't be above hot-gluing a razor blade to the end of a popsicle stick to achieve the same thing.

Here is a link to the foam:

https://www.fastcap.com/product/kaizen-foam

And here is the scraper ($11 US)

https://www.fastcap.com/product/kaizen-scraper

You are completely right! You can also ask someone who is making flight cases for keyboards or other instruments ...

Is a possible lateral thinking solution for that to use (as few as practical) multiple layers thinner sheets, each layer cut as relevant for that level?

e.g.
A base layer just cut to the plan shape of the case. Another the same external size but with a full 'Stick shape' cut out of the middle except for the recessed part between block & headstock. Another the same external size but with a full 'Stick shape' cut out of the middle. Alternatively have a simpler single 'upper layer' and handle the recessed part of the beam with a slimmer insert?

Just thoughts without having done any research on local foam availability or having a real case or Stick to look at yet. I imagine the belt hook maybe be a complication, but at this stage I can't be sure?

SteveS: Surely there must be an equivalent of this sort of website in the US?: https://www.efoam.co.uk/packaging-foam.php

These UK people do various foams in various forms for various purposes, including the intriguing "pick 'n' pluck pre-cubed foam" and the softer "convoluted" foam you often see in the lids of cases.

The "Pick & pluck semi-rigid Black Ethafoam" about half way down this page looks like a very flexible (in the configuration sense of the word) option & comes in 50mm or 25mm layers.

Lots of possibilities, it seems.

There sure are, David. The challenge is finding someone that sells in quantity and size suitable to the purpose, & doesn’t rape you on shipping. The piece I ordered was going to cost more to ship it than the cost of the foam itself. Purchased (from the same vendor!) through Amazon w/free shipping so it was about $65 total. 53” x 16” x 2”

Note that the pick ‘n’ pluck foam is certainly convenient, but again that is the same polyurethane foam that deteriorates and crumbles over time. The pick ‘n’ pluck solves the problem of cutting it - the polyurethane is very soft & difficult to cut cleanly without the special tools. However you can get a kit of a set of heat-cutting tools for real cheap (under $30 US).

Also your point about multiple layers to eliminate most of the need to level the bottoms of the recesses is spot on, though it is necessary to cut a recess in the bottom layer to accommodate the belt hook. However if I do it right I think I can just forego the foam entirely under the belt hook. If the bottom of the stick is supported high enough to give a half-inch/12mm clearance between the hook & the bottom of the case I should be good. The sheet I just received is actually 57mm thick, almost 2 1/4” so two full layers should fit perfectly inside this case (4 1/2”).

The sheet is big enough to fully fill the case, though I’m going to do a three-block support so I’ll have enough left over to make a case for the Railboard (I’m planning on making it look like a vintage Fender tweed guitar case! )

I’ll post some pics in a new thread when I’ve got the foam cut & installed.

#1642 is now in it's new home. Arrived yesterday. As it was thoroughly cold from several days in transit I resisted the temptation to get it out right away. I just removed the outer wrappings & sat the unopened flight case in my den/music room overnight to acclimatise.

Opened this morning. Every bit as beautiful as in @PunkPonyPrincess's pics.

The action, especially on the bass side seems high (strings 7 & 8 especially so, 3 to 4mm!) so an early job will be discovering how to improve the setup.

I'll probably have questions...

PS. I see from the 2020 sale of #1642 that the owner at that time had tried to rearrange the bass side in descending 4ths, including swapping the screws round, then swapping back before sale!