It's not so much a question of getting them to dry flat for me; it's getting them flat after they are dry. I air dry my prints on screens in the basement--so they do not get bone dry. Then, what I do is interleave the prints in clean blotter sheets; place them between two large pieces of 3/4 inch plywood I bought and cut for the purpose; then stacking books on top for a coupla days. Low tech, but it works well.
Ferrotype tins are for glossing.