My semi-rigid resin should be arriving soon! In the mean time I thought I'd have some fun and try casting some hot melt. As expected the result was a bendy version of the part. This isn't particularly useful for this project, but it might be a good approach for other sorts of parts.
I wonder if there's something that can be mixed into hot melt to make a more structurally sound composite material. I've done some searching for
On the bright(ish) side, I discovered that my test mold has a significant issue where an air bubble bridged an important part, so I'll be able to re-make the mold before I start using the resin.