Hey Zachary,?
I am dutch but to keep it readable for the rest of the people as well, I will keep it in english.
When it comes to all your glassfibre and filling needs I would like to recommend polyestershoppen.nl, here a direct link to the filling materials:?
For the plywood it is best to contact a woodsupplier like the Arnhemse houthandel:?
If they can't supply you with the right size you can always make a buttstrap or a scarf joint. Buttstraps are easiest but create a less flexible area around the joint, scarf joints seem a bit harder at the start but when you would make a scarfing jig for it(possible for circular saws or routers, whichever you have handy or you prefer) but at the plus side, with a scarfjoint it looks a lot better, is stronger overall and avoid the need for buttblocks.
If you would take a look at zeilersforum.nl, there is a member there that started making his 34 foot catamaran in a 40 foot shipping container, I believe his/her username there is ameezing.
