Wood = weight and future rot. About the only thing you can do is treat the wood to delay the rot. Marine plywood is best due to the fact that it is glued and pressed with waterproof glue, and usually has a quality wood used in the manufacturing of it. That said, Marine plywood is also very expensive. If you buy a decent 3/4" BC plywood and treat it to seal as good as possible, it should give you years of service. Especially if you keep a cover over your boat, or keep it garaged when not in use. If at all possible, try to use aluminum for any interior support bracing. It's lighter, and does not rot. 2 x 2's, 2 x 4's will work just fine, but will add a lot of weight which will slow down your rig. It's a matter of budget, tools and knowledge that should make your decision. Either will work fine. Make sure you use all weather type fasteners when putting it all together.