Right now I'm doing 4x5 contact prints onto 5x7 enlarging paper, and soon I'm planning on doing 5x7 contact prints onto larger enlarging paper (and eventually I would also like to try some traditional contact printing paper but enlarging paper has been easy and available).
I've been using a Proof Printer which has been good and easy - arranging the negative on the paper is a breeze and the glass is very heavy and keeps the two pressed firmly together, but I understand this is not the way things ought to be done.
Why use a more expensive and cumbersome contact printing frame?
Contact printing frames have a hinged back, this is needed to inspect the image exposure with Print out papers, Albumen, salt prints etc. You need to keep the negative in register with the paper in case you need to give more exposure.
The contact printing frames that I have used exert more pressure on the "negative/paper sandwich" than what could be achieved with the glass alone on the proof printers that I have used. If you suspect that the pressure is inadequate to make a sharp contact print, you may want to try and to increase the clamping pressure, or use a different type of printing frame.
Oh, making a contact printing frame with a spring-back is fairly straight forward.
Yes that is a great contact printer as is a sheet of 1/4 inch glass. you just can't look without changing the register if you are doing alternative processes with UV light.