Gummed paper tape; the kind used by watercolor artists to tape down their paper. It has gum arabic (or something similar) on one side, much like old-fashioned envelopes or stamps. Squeegee or blot the print, put it face-up on a rigid and flat support (I use a couple of old mirrors), then apply the tape all around the print. On FB paper the tape needs to overlap the paper by 5-10mm or so depending on print size. Then allow to dry; the print will pull itself perfectly flat as it dries. When it's completely dry, cut the tape along the edges of the print. The print will end up having strips of tape all around the margins, but these can be overmatted when framing, or simply trimmed. So you need to allow for this margin when printing.
I have heard of some modest success using removable painter's tape on small prints in a similar way, but have never been able to replicate it. Plenty of people use the gummed tape approach; it's one of the 'default' ways of flattening FB prints and works very reliably with a minimal requirement in space and investment.