A 3D 'Boatmate'? You've drawn the short straw there, Doc. I use a flat board, like most of the folk who've replied. For other assemblies I make up jigs ad-hoc and keep them only if I think they might be needed again.
I really wish I could help. because I've lost sleep thinking about this for you. I've concluded that even if you could design one you'd never ever manage to compile instructions on how to use it which would be intelligible to yer average boatie – even if they bothered to read them. It would also be so expensive that only a very few would buy one. By "Expensive" I mean more than a tenner. At that price both teeth and breath tend to be sucked in.
I had to retire my Workmate 400 when we moved house last year. I'd bought it shop-soiled from House of Holland in 1976. I think I paid £11. I shed a tear…
It's not a crime to withdraw gracefully. Check what W C Fields said about it. **LINK** Would that our politicians would accept that.
BTW is "jigs" a shortened form of "jury rigs"? One for the etymologists out there.
Dave M