Temporary? Have some fun with that fact and throw a rug on it 
Especially funny if it's just an oval on a nice oval throw rug (I used a soft "spiral rope rug" on a table outside and it was near silent and looked the part. A rag rug would be equally "home-y" imo.)
You noise is comming from different sources too, reflection off the top; transmission to the layout/"drumming", and it's echo from the underneath. Not screwing track down tight, isolation layers (foam/homasote), and heavier skirt curtains help with the underside noise. Top reflection will be based on your scenery and ground cover. Smooth and hard will be louder, static grass and soft moss bushes everwhere will tone it down.
Carpet is actually one of the best sound insulators. Comming up off the floor you'd expect might quiet it some, but it is the opposite.
Cross braces on the frame help with drumming too.
. If you aren't too worried about realism indoor/outdoor AstroTurf is great and hardly allows any shifting. My pal used it and I wasn't keen on the idea till I saw and heard it. A short nap rubber backed resteraunt/bar carpet would likely be best though. Just enough nap to prevent shifting, and short enough loose fibers arent wrapping tour axles. (high quality will help on loose fibers too)
Pin your track from shifting vs screwing down tight. Tight screws transmit more vibration to the wood. Or use small amounts of caulk to glue it. Nylon hardware is another option. Rubber washers would help some if you must screw it down tight.
Foam works, but I was kinda expecting it to work better. For below grade/ terrain carving it cant be beat beat though; makes it very easy and carving mountains, etc. you almost can't mess it up bad carving or painting....it's that easy.
I've wanted to experiment with no nap carpet sandwiched between plywoods or foam. I think that would just leave the sounds reflecting off the top.
