Why don't you route your track in foamboard? That's what I finally did after numerous tests and it's wonderfully light, stiff, durable and easy to handle. Three sheets glued on top of another: one 1/2 in for the base, and one 1/8 and one 3/8 in both routed for the track itself, which gives about 8 mm depth. You don't even need to paint, since adherence is fairly good with urethane tires.
