...Just got a little cooler.
I know, because I built it.
It's an old catering cooler... The really thick styrofoam type. I took 3/8' pine and built a nice crate around it. It cost about $50 give or take, but I've noticed that one thing that snaps spectators out of the illusion of an 1860's military camp is something modern and out of place... ie, a big red Igloo cooler.
Now, when it's closed, it resembles an ammo crate or an old hardtack box... It leaves the illusion intact.
Yes, I realize that the troops back then didn't have coolers, but if they had, they woulda used them. The modern reenactor, bringing history to you with luxury.