This is my first entry into a competition, and it's a hexagonal coaster with a hexagon pattern imprinted onto the top. 

The raised edges around the coaster allow for condensation to pool rather than dripping all over the table, and the bottom of the coaster is angled equally so that it can stack easily.

It should work at any scale, and rotation shouldn't affect it, as there aren't any steep overhangs that require direct cooling

Because of its large surface area however, a brim is recommended, unless your bed has excellent adhesion