I travel doing BBQ competitions and made a plywood "pig box" like you describe to transport whole hogs. Worked OK but finally got tired of repairing it and looked for a better idea. Found a used single lid aluminum truck tool box ($35), placed 3" foam insulation on all sides, added a drain from the marine store and then had it Rhino-lined. It's SUPER tough and with some home made ice blocks in the bottom will keep a pig cold for a week and it's lockable to boot. The Rhino lining is food grade plastic so no worries there. I've since made a smaller one and I promise it's every bit as tough as those $500 Yeti roto-molded coolers and a whole lot cheaper.
