Maybe I missed something, but I'm surprised that I haven't found any mention of the method we use quite often here in this part of the world. After washing, put the print face up on a plate of glass (with appropriate thickness) and fix the border with an adhesive tape, the whole perimeter, about 5mm on the print should be sufficient. It must be the old fashioned brown paper tape which is usually licked to get wet (or passed over a wet sponge, but do not wet it too much). Let the whole thing dry until the next day (don't hurry too much) and then cut the print with the stuck tape from the glass and then cut off the border strip with the tape (or hide it under a passe-partout). Especially with "F" papers the surface will get a very nice smooth look and will be perfectly flat.
The forces which try to contract the print when drying may be quite big with large prints, that's why the glass should be thick enough (5mm for 30 x40 cm will do).