Fencepost,
For "sticky" rubber mats, look at the soft rubber mesh that stores sell to line kitchern drawers. Try stores such as KMart, WalMart and Ames. Sears sells a heavy duty version in the Tools section. They sell it as a drawer liner for the big roll-around tool boxes.

If you are at all interested in something sturdier, try searching on "laptop" and "vehicle" and "mount" in these forums. Several threads have come up before. The solutions range from homemade to professional designs.

Cheers,
Thorn