In our first real attempt at a track, TheFanMan and i did this: we used sound isolation board (2x 2-4' sheets) that can be cut pretty easly. we then used 1/32" plywood strips about 1 1/2" tall and, using a ruler, pocket knife, bottle opener (haha..be quiet) we set them into the track about 1/4th inch. See a pic here:
http://www.kassatsu.com/Enduro3.htm