I've been pinning various 1/35th scale dioramas of LAV IIIs for how they have done the stowage, and I notice bright red or blue plastic coolers are a common item.
Like this nice model
from this blog. Notice it has two, a red and a blue!
I also spotted one on a platoon of '80s Cold War M113s over at The Guild gaming forum.
Much of my stowage consists of wooden boxes, which are rather rare in these days of cardboard ration boxes, cases of water bottles and plastic coolers.
I do have some metal boxes which perhaps could have their corners filed down and then painted blue with a white lid to look like a Coleman cooler.