I am printing a portfolio right now for a photographer , 50 images 2 - 20 x24's each, this will not be mounted and I have been worried about **flatness** this for the period I have been printing this rather large project.

Solution
I have a controlled humidity in my darkroom, so when finished I will pump up the humidity and store the prints in the room over a couple of days, then I will move my smaller hot press into the darkroom and carefully hot press each print and then move them out to normal humidity conditions , in sleeves and in boxes to hold them flat.

This will probably be my new approach to print flattening.
In Toronto Ontario, we have four distinct seasons, during the winter months lets say from Nov 16 to April 15 the humidity drops very low and give us printers a huge amount of problems, therefore I have humidified the whole lab to counteract this damm problem.
During summer the normal humidity is high and things go easy, but I print year round so we need this solution for 6 months of the year.