Just completed a subwoofer platform....
actually 2 platforms for twin Rel Storm III subs. I couldn't spike my subs directly to the subfloor of my condo because all the energy was resonating through the floor so I tried placing the subs directly on the carpet without spikes, not good, flabby bass and ill defined.
The only way I thought I could fix it was to spike a custom made platform directly to the floor for rigidity. I could then place the sub using spikes directly on the platform which has custom machined ABS feet and brass footers which prevent the subs' energy from going through the floor. The assembly is very rigid and the sub is grounded very well to the platform. I tested them out and sure enough, no energy going through the floor and tight, well controlled bass, better than I have ever experienced with these subs and I've tried many different methods over the past few years.
To be fair, I have my own machine shop so all of the items fabricated were machined and built precisely so the DIY factor is perhaps overstated but none the less I wanted to share the idea and results for those interested in obtaining better performance from their subwoofer.