"The only modification I’ve done to it was drill a couple of holes on the bottom to drain any water." There is a petcock you can buy for turning barrels into rain barrels, I would have used one of them so your box could also be a rinse tank :)
I’ve bought stainless hardware from Lowes for my back plates and camera. Found the fittings to turn an old tent pole and fishing float into a travel size dive flag float that is just bouyant enough that I can pull it under when going under docks, boats, anchor rodes and fishing lines. At West Marine they sell stainless cable and the fittings and I used their bench mounted swagging tool to make custom lenght BC dump cables.