Never had a problem with pop rivets coming loose if they are done properly and remember a nutsert is a rivet with a screw - 2 parts to come loose and 1 part to get in the way.
I'd make the pads a little longer and use 2 track nuts (1 front 1 back) to hold it on. Or just get some track mount feet and attach them to the underside of the rack...
If you are putting batteries in the tub DC-DC converter is the cheapest option what you pay extra for the converter you will save in cable costs and early battery failure prevention...