I wanted to also use these filters with my pinhole camera and multigrade paper. The contrast with the paper negatives can be a little extreme and these filters can tame contrast. However, my filters can't easily be taped to the front of the camera. I had to devise a method to hold them.
The 3 inch filters are designed to go under the lens on a darkroom enlarger. Each filter is mounted in a plastic holder that slides into a corresponding mount attached to the enlarger. I pulled out my calipers, did a little measuaring, and crafted a design in Tinkercad. A hours later and I had this design:
I decided to print it in two pieces on my Monoprice Mini Select and use a fair amount of cyanoacrylate. The grippy bits were printed with the flat side down and the larger filter holder was printed standing up. Because of the tight tolerances required in the filter holder, I didn't want to deal with internal supports. There would be too much friction when I try to install the filter.
This camera has a 90° angle of view and several iterations didn't take this into account. In the end I guessed about the diameter of the cut-outs. The corner of a sheet of paper was a perfect tool to see if there was any obstruction in that 90° angle of view. It looked good.
All of the filters slide nicely into the holder and the fingers grip the bezel around the pinhole. It's sturdy and easy to use.
The next step is to try out the filters with a paper negative. Maybe I'll be able to tame some contrast.