It is easy to flatten fibre prints. Let them dry and they will curl in all ways and this is normal. Put them face down on a dry towel and hold the left edge with your fingers. Use a straight edge on the back of the print and pull the print to the left under the straight edge. Most of the curl will have gone. Do it as many times as you need and the print will be almost as flat as a resin coated one. Just right for mounting.