Double Marble Puzzle

Designed to be a Christmas gift, this is a half of Double Marble Puzzle based on the original design for woodcraft made by tomatoskins and made avaliable on instructables.
Print two copies of the thing and glue them together, put the marbles on the "valley" made and glue a screen to stop the balls from scaping.
The goal of this marble puzzle is to be able to rest the marbles in the holes on each side of the thing. Upon first contact with this puzzle, many observers will try to tilt the block