I have 2-150qt marine style coolers, borrowed another 3 big coolers for my bison, but had thought about building one in the bed of the truck.
Thought about buying plywood, and that 2" sheet insulation.
If you had a table saw with a dado cutter/blade you could have the pieces of insulation interlock with one another.
Some guys just buy/use one of those deep chest freezers, that may work even better. Just get a ac/dc converter, and/or maybe some dry ice.