This was made from a scrap piece of Plexiglas and a sherline mill.  The surface is smooth but I haven't noticed any slipping of the camera when it's attached.  It could be made of wood with a 1/4-20 nut imbedded for the tripod.

The slot allows the tripod socket to be placed at the nodal point of the lens.  I can drill more holes and tap them for other cameras if I want to.

Here it is on the tripod with my D490 taking a picture of itself in a mirror:

Just for fun here it is attached to my old Epson PhotoPC:

Here's it attached to the Olympus C-2100: