If you visit your local HVAC supplier, you can usually find the same Neo/Cork feet for a buck a piece. I know Mapleshade uses exactly the same foot, sticks two together, then trims it so the sides are evened up.....and of course sells it as an 'Isoblock' for MUCH more. They do work well in a pinch, but by FAR the best solution is to go inner tube/MDF/roller bearing....that way you satisfy vertical as well as horizontal vibration issues.
Kenda makes inner tubes for $3ea.....Shephard makes wooden furniture cups P/N# 3697, get some steel/tungsten carbide 1/2" balls.....and you're set.
Believe me, you'll be astonished.