You could buy a rubber workshop mat from a tractor supply, home depot or similar place. Cut it into little squares to fit the pad frame and stack them until you have enough. You will probably need 2-3 square feet of mat per side. Attach using existing holes if possble or drill your own and use a 5/16" carriage bolt with a fender washer on the bottom of the pad. As you tighten the stack, the carriage bolt head will not touch the concrete. You should be able to make your own this way for $35 or less. It would be easily removeable. If you wanted to get fancy, you could glue the layers together
___________ < pad frame with nut on top
PAD | PAD
PAD | PAD < bolt going through layers of rubber mat
PAD | PAD
------ < fender washer
[] < bolt head