I built my box out of 1/2"x10" pine planks all around. Box is close to 12' long, 4' wide and 1' high (limited by Dolphin Striker clearance). The front 4' has a lid that slides backwards on standard home depot draw slider rails. The rear 7' section has a hinged lid that I prop open with a leg that is hinged to the lid and comes to rest on the side edge of the opening. I just lift the lid a bit to disengage the prop leg, push the leg up to the underside of the lid where a magnet captures it and holds it up & out the way when the lid is closed.

I uses cheapo polyester resing from Depot to cost the entire structure inside & out. Used some cloth in the corners and plank joints to make a good water seal.

Been very happy with it.

Would not use thin ply as a skin over a frame...did this previously and it all came apart within 1.5 years (but I didn't use and glass/resin on it).

Mike.