I have a frame that comforatble holds 11x14 negatives. It came out of a mid 70's plate burner.
It has glass plates on both sides of the frame , and has a piece of black 'velvet' bonded to both sides of some kind of cardboard board that sits between the glass panes.

Originally the frame was hinged mid long edge to swing in the middle on a support so that you could load one side, then flip over and load the other side. I guess you flipped it while the burner was on so as to do two burns for one start of the arc lamp.

The velvet is stiff to crush, and keeps the sensitized paper in good contact with the negative up against the glass.

I have yet to use it by loading two sides at once.