I just picked up a Blackburn 20lb bike rack from Walmart and and had the thought of putting a single one of the large plastic Harbor Freight ammo cans on the back. It's going on a Schwinn AL Comp, 2020 version. Now I'm considering ordering one of the Rockbros racks, as they seem to have better support and are rated much stronger, and then I could put 2 of the large ammo cans on. Main use of the bike is off the road, traveling trails in state forests, so abuse resistance is a top priority.
@Ravenbar's Repairs: The only issue I have is getting my leg over the ammo can. I can do it, but it takes some acrobatics. I do like that I can padlock the can shut when I go into a store and leave my bike tools and amateur radio gear I have safely stowed.
