B:
I would HIGHLY recommend NOT using the floor as the bottom of the keg box. Most earth contact structures remain at 52 degrees and trying to make the floor colder is like trying to cool your backyard in the summer by keeping the door open and letting the AC out. It’s a loosing battle.
What I might recommend is to lay at least one sheet of insulation and a 3/4″ plywood floor between the basement floor and the keg. You can use thinner stuff than the ProPink 1.5″ thick stuff. I see a few people using the 3/4″ insulation with a thin reflective aluminum film on each side.
Not sure what the R rating is, but it would be better than nothing.