I might think a thin layer of foam of other elastic material on top of a sturdy piece of plywood (maybe 3/4 birch) would help to take up any surface irregularities to promote solid contact. One would want relatively thick glass (or maybe acrylic) so as not to have it explode from the force. Wingnuts and bolts, or some toggle clamps, could be used to clamp the frame.

Heh, I think I have a 2 1/4 x 3 1/4 Kodak contact frame somewhere that's been in the family since 19-ought-48 or so. (Velox and a Tri-Chem Pack anyone. )