My grandfather was a furniture maker/restorer. He always made a secret compartment when space and design allowed it. The compartment that I remember best was a small wooden box made out of 1/8" stock where one surface stuck out about 1/4" on each side. The pieces that went on the furniture were 1/2" square stock with a 1/4 by 1/4 section removed along it's length. There were 2 of them. They were attached to a vertical part of the furniture at the bottom of the unit. With them in place the compartment was slipped in place and that's it.