Unless the thing is walking across the floor I wouldn't worry too much about putting anything underneath it.
Now, if you want better sound -- you need to get into mass loading. The less the sub vibrates on the outside, the more energy is delivered in making bass. The energy is there, and you can either have it shaking the cabinet, or thrown out the driver.
I used to make my kids take turns sitting on mine. I would incorporate "time out" and better bass at the same time.