I have two Gramma pads under my SVS NSD 12/2 (one wasn't big enough but two placed sideways and one in front the other in back made the perfect fit for my large sub. All I can say is the 60.00 I paid for both (from a S&V forum member) was well worth the money.
I have a huge great room that is covered on both sides by bay windows and sliding glass doors and windows and everything in that room rattled, but once I put my sub under the gramma pads, all rattling stopped and now all I hear is pure clean bass.
You could build your own, but as mentioned finding the foam that can handle the weight of the sub and properly decouple ths ub is another thing all together, best to buy the Great Gramma and be done with it, one of the best investments you'll make to improve the bass.
I'm just pissed at myself that I waited so long to buy my pads.