I recently had some student teachers come and do a collagraph lesson
with my students. They coated the "plate" with glue stick and put
plastic wrap over the mat board plates once everything was placed and it
worked very nicely. I'm sure you could add color over the plastic wrap
with permanent marker or acrylic paints. We didn't try it!
The students' teacher also said someone she knew who didn't have a
press, put his plate and cushion between two pieces of plywood and ran
over it with his truck - seemed to work beautifully.