Foxfish,
For help with the shape and design, you can chaeck the following sites. omer makes a cracking wood gun - Ive tested one, and believe me, if you manage to do it, I will even buy it
Check:
Totemsub . arbalètes en bois et videos de chasse sous-marine
Bricoler une arbalète en bois pour la chasse sous-marine
OMER Arbalète COBRA CIRCULAIRE 130cm avec trou sandow rapide
If you need some help with the french, just holler. There was also a crazy guy in New caledonia that made spearguns by measure, in some kind of tropical wood. this was a few years ago, i will see if I can remenber and will get back here. Not laminated, whole piece. It sounds that you are embarking in a great project. Let me know how I can help
Best,
Oak