Decades ago in another professional life I used the following non-framed top-to-bottom layers:
* Oversized sheet of heavy 1/4-inch thick glass treated by me for ANR, treated side down.
* Glass plate negative, emulsion side down.
* Photo paper, emulsion side up.
* Black foam or neoprene base pad.
The foam/neoprene base is a softer pad, able to compress sufficiently so that the glass negative does not crack, yet rigid enough to still force the paper flat.
The edge-beveled (for safety) heavy glass sheet is ANR-treated by first cleaning it, then spraying a very light overcoat of clear hairspray onto one side. That side becomes the ANR side. The microscopic dried droplets supply sufficient offset to prevent the rings, but do not show in the print.
Gravity holds the layers together. Obviously no clamping due to the delicate plate negatives. Film negatives may be clamped by adding a rigid board under the base pad. And not exceeding the cracking strength of the top glass sheet.
This is how I currently contact print my 8x10 film negatives under an enlarger. At least until I can convince the other half that a vacuum contact printing frame really is a must-have...