Another approach could be to bend the wet paper over the edges of a smaller
piece of plexi after sizing and fix it mechanically by strong clips. It would
stay on the plastic until the print is ready and should dry to the same
dimensions after each coating/developing step. (Longer drying times, probably
not so easy to handle as it sounds like: s. th. for tinkering freaks...
I prefer all-rag papers for gum and if they had shrunk too much, I simply
moisten them from behind with a moist sponge or towel until they have expanded
to the desired size again.
Klaus Pollmeier