Pictureframe keyhole hanger insert

thingiverse

This keyhole hanger can be inserted in a hole to hang pictures or lightweight furniture. Depending on the drillbit and the softness of the wood you might want to oversize the hanger a little bit (e.g. 20.2 mm for a 20 mm hole). For super-critical stuff you might want to add a drop of epoxy to the side before putting it in the hole.