I also tried one from Photgraphers Formulary and hated it. It never worked correctly and they tried to send plastic clamps to replace the metal ones that didn't work. It was poorly conceived and poorly constructed. And, as mentioned, physically painful to use.
The best one I've ever used was from Doug Kennedy. You can google and find him. Excellent in function and appearance.
my experience with contact printing frames has been good in some regards and not so good in others.
I have three contact printing frames. one very old 8x10 contact printing frame... Mateo mailed it to me about a year ago and it was what I used to do my first 4x5 contact prints in silver and cyanotype. It works amazingly well even with how old it is.
The second one I got was a 11x14 photographers formulary frame. it was great at first... lasted about 4 months and the clips/pressure springs along the edges started wearing out and eventually 3 of the 6 clips completely disintegrated.
after that, I bought a 16x20 frame from bostick and sullivan and after adding some foam kitchen drawer/cabinet liner and felt it works wonderfully for everything from a couple 4x5s all the way up to making 7x17 contact prints. The foam and felt helped to create enough added pressure for all the different sizes.
so from my experience Id steer away from the formulary ones (no offense to the photographers formulary - just seemed a minor design error) and get one of the bostick and sullivan ones because the spring design is really nice and works well.
The pelland frames seem very nice as well. I spoke with Dan Pelland and he quoted me a very reasonable price on getting a custom designed frame for 7x17 contact printing.
Doesn't a simple glass plate do the job?
I place my negs on the paper, press the glass plate on it and give it some light, can't be easier.
I just use a sheet of 10x8 foam rubber as the base, and a piece of 9x11 glass (tape around the edges).
The 10x8 foam rubber helps with placement when doing colour prints in total darkness (i just feel for the edges and align the paper on them), and then just plop the sheet of glass on top.