When I was in high school, I made an isolator that worked very well for my turntable. I've wondered how well a larger version would work for a sub.
Basically, I made a nice-looking wooden "pan" (say, 16"W x 16"D x 2"H) and filled it with sand. I placed a "floating" panel on top of the sand (For the example above, it would be maybe 15.5" x 15.5") and sat my turntable on top of that.
So, the TT was very well isolated from rumble as it was essentially sitting on 2" of sand and not touching anything else.
It's certainly quicker and easier to order a GRAMMA pad, but I've always wondered if my little device idea would work for a sub.