On a table or desk, I have a wood block 4x10x1/2 with a 1x1x10 nailed to one flat side. I put the MBP's hinge side on the bigger block, which raises it off the table just enough to keep it cool. The 1x1x10 is slightly off center and keeps the MBP from sliding around.
Cheap and it works.
I'd think the magazine suggestion would work well on-the-go. Otherwise, you're carting around a pretty cumbersome piece of equipment.