The advantage of making a custom case can also be a problem. The liquid foam is "blown" around a specific instrument, so unique to that design. This means that the horn can't move at all, which is extremely good, but any other horn, unless exactly the same brand and model, won't fit.
I grow giant bamboo (along with many timber species, as that's my work... forestry research) so have that resource. However it requires first cutting, then curing, which takes 3 to 6 months depending on whether it's winter or summer. The easy parts are the leather hinges, straps and lining.
Everything is possible, however not cheap, easy or quickly done. If you have sources for giant bamboo I suggest you commission someone to make one for you that is local to you. The instructions for construction are in this thread on page 2 # 26 and a supplemental on this page #43. It's also possible to make the case out of a solid block of wood that's cut to make hollow and glued back in pieces using a very large bandsaw. Cylindrical forms tend to be very strong. A rough way to do this using a block of wood is to cut in half and hollow out with rotary files. It will be heavier and a lot more difficult than just building a "box". A cheap and nasty version could be made from large diameter PVC pipe and would be very strong (but ugly).
Best of luck!