I built one of plywood on a 4X8 flat trailer, but as I have no project skills it turned out like most all my JH Shop class assignments did (a mercy "D" just to get me out of the Shop teacher's hair, I thank God I wasn't required to take Shop in HS...).
After using this abortion a couple years I bagged it and bought a 5X10 cargo trailer. Best goose hunting move I ever made, should have saved what I wasted on the plywood job and done it sooner.
The plywood box didn't stand up to the bouncing around of field hunting, and the jostling the decoys took was bad, too. Decoys are just too expensive to trash like that. Absolutely no question the couple seasons being stored/transported in that trailer took years off those decoy's servicability.
Anyone with a screwdriver could have breached the high tech hasp & lock security system. If the trailer can't store & haul gear/decoys while keeping them reasonably protected from damage or theft, a guy is just wasting his time.
With the cargo trailer my decoys & gear are organized, don't smash & bounce around, and are about as protected from the elements & theft as is possible. I not only carry more decoys, but the organization system I came up with allows me to set up and take down a larger spread in much less time than I could using the home made trailer.
After I got the cargo trailer I took the plywood mess apart and now use the flat trailer to haul my motorcycle...