I use a sheet of glass to proof print a whole roll of 120 film in transparent HAMA pages on 24x30cm paper. Works perfectly.

for contact prints of a single frame I have black paper insert masks which fit in a 9x12cm contact printing frame: One for 645, one for 6x6, one for 6x7, another 6x9, and a 6.5x9cm sheet insert where the whole film sits in the opening in the mask (with rebates and everything).