Once we had a failed print on a regular cube, but realized we couldn't stop playing with the broken one - the opening was so interesting. So we broke one on purpose! We took out  a hinge and added magnets. 

Magnets are easy to add - magnets on the same side should be the same polarity, while the pair on the other end of the open cube should be the opposite. 

We use 5x2mm magnets and press them in with a drop of glue. Let glue sit for several hours (24 recommended - be patient!) and enjoy.