3/8" plywood is nowhere NEAR thick enough; for one thing, it will flex with just the weight of a drum kit unless your framing is maybe 8 inches apart or so; also, that thickness will resonate at even higher frequency; it's all about MASS, and you don't have much.
Sorry, sonic, but he's not talking about a continuous sheet of rubber; if so, the riser would only be 2' x 3', the size of the piece of rubber. You'd be lucky to get a set of BONGOS to fit on that. Also, with full coverage even soft FOAM wouldn't compress to 50% with only the weight of a drum kit and person, much less neoprene...
Shout, if that's all the thicker platform you have you're not gonna get much good out of building a riser; there's not enough strength in that plywood without having frames REALLY close together, and even so it would ring quite a bit - this would make your sound WORSE by adding tuned resonant notes to the mix.
Adding carpet UNDER this thin ply, if it's actually a sandwich consisting of the original floor/carpet/ply, shouldn't HURT the acoustics but it won't HELP either. Also, if you leave it for long periods you'll grow all sorts of interesting molds, fungi, and critters - carpet needs to BREATHE and be vacuumed to avoid this.
Overall, I don't think you're even close to the right approach yet, sorry... Steve