I use blotter rolls, but I don't think you can buy them any more. This is two sheets of blotter paper between sheets of soft corragated cardboard. You place the prints between the blotter paper and roll the whole thing up with the prints rolled emulsion side out. I still have three of these.
I also use blotter books which are hard to find. See the attached photo.
Another alternative is to use print flattening solution. A formula is listed in the "Darkroom Cookbook"which is simply 2 oz.of glycerin and water to make 32 oz. or see the second attachment. This is from B&H
These are things we used 50 years ago to flatten prints when there wasn't RC paper. Hard or impossible to find today!
I see the photo of the blotter book didn't work. B&H has these also.