I have yet to use my enlarger, I have almost everything I need now including 2 Nova print process tanks etc.
I store my negatives in the more expensive Kenro A4 acetate sheets which you can print through without taking the negatives out (pain!)
Obviously the slotted contact printer devices are of no use, so I was thinking of simply buying an A4 acrylic sheet to set on top of the negative sheet.
I was thinking of 10mm think so there is plenty of weight to flatten the negative sheet out.
Acrylic will scratch easily. Get either a glass with smoothed/rounded edges at a local glass shop or tape the edges of a pane of glass from a picture frame.
I started using a piece of plate-glass, then managed to have the box containing it dropped during a house-move. Since then I have a piece of 5mm polycarbonate. For normal contacts (ie. as a reference, not as the final result) they both work adequately, apart from the plastic being lighter in weight and needing some help with a couple of fingers to keep the film in good contact with the paper.
I'll call into the local glass shop to see if they have something suitable lying around. I thought of using picture frame glass but it is very thin and light. They make a lot of furniture using thick safety glass with bevelled edges and they are always very helpful.
Yes, use glass. The 1/4 inch thick kind, sized somewhat bigger than a contact sheet (mine is about 11 x 14) will have plenty of weight to hold negatives flat. Be sure to have the edges smoothed.