A couple of packages of weight-matched soft feet. Two packs as the subwoofers will be stacked.
This is where they end up, of course
One string shelf made
And insulation going on top
And if it isn't obvious, there's about half and inch of clearance between subs and walls so they can theoretically move freely on the feet even if they don't (above 3-4 Hz or so).
Ran out of this kind of insulation, so I can't do more today.