I did some research because I will need it for mine. Here is a link to a place near you that has 5' wide marine grade plywood. It is not cheap but no seams and made for marine water rich environments.
http://www.houstonhardwoods.com/hardwood_products.php
I am going to put a slight radius in mine to shed water. Would like to do aluminum to keep it light but radius might be a problem. Alternative is two layers marine 3/4" for main cross pieces, ends and sides, fit to just overlap the exterior of the bed. Hydralic lifts (minivan rear door style). If you look at manufactured ones you can get a sense of where and how they place them, I have used mini versions on the hope chests I have restored for my wife. Haven't figured out the locking yet.