Lacking a dry mount press, I'd struggled for years to get flat FB prints. And then I made a film drying cabinet that also permits drying small print within. Then I hit on the idea of taping FB prints, still damp after squeegeeing and partial drying with a hair dryer, face up on sheets of glass, using drafting tape. I apply the tape to the border of the print, not the main image area (i.e. covering the outer 1/8" or so).
Viola, I let the prints dry completely inside the cabinet, and they really are remarkably flat. I find it helps to check them about several hours into the drying process, and re-secure the tape along the edge of the print.
After drying, I carefully remove the tape by slowly pulling it off with the tape being pulled toward the edge, to stop any potential tears. So far, I've had no issues, and there doesn't seem to be any residue that I can notice.
~Joe