@danec's "Plasticlamp" system is awesome.  This part allows Plasticlamp stuff to mount to a camera mount, which has ¼"-20 thread.  The main block has a recess for an M3 nut, a slot for a ¼"-20 nut, and a hexagonal inset for connecting to Plasticlamp.  If, like me, you want to connect it to a Plasticlamp piece that also has a hexagonal inset, there's a 4mm hexagonal piece to join two inset pieces.

Should print fine without supports, oriented with the slot for the ¼"-20 nut facing up as in the file.

Insert the M3 nut through the hole opposite the hexagonal inset.  Press it down as far as possible (it's a tight fit).  Attach the adapter to your other Plasticlamp items using an M3 bolt.  Insert the ¼"-20 nut into the slot and attach the adapter to the ¼"-20 mount.