An alternative to the 42mm push-on filters is 25.5mm screw in filters. A screw in filter allows you to adjust the aperture without removing the filter. Also less likely to fall off!
Although you can easily get a Pentax 25.5mm UV filter that was originally made for a Pentax 110 camera, b&w contrast filters do come up every now and then. I have a Minolta 25.5mm filter in y48 (yellow), and several filters made specially for the Super Nettel. The Super Nettel filters are very thin to allow the camera to fold closed with the filter in place.
You can also buy a Hoya 25.5 UV filter and replace the UV glass with a disk cut from a larger filter of whatever color you like. The Hoya filter holds the glass in place with a threaded retaining ring that is easily removed with a spanner wrench. I bought three of these but haven't yet switched the glass. The more commonly available Pentax filters appear to hold the glass in place with a pressed-in aluminum ring that is not easily removed. The Minolta filter also has a retaining ring and is a little thinner than the Hoya, but I have seen fewer of them.
I realize that the above begs the question of how to cut the correct size filter disk from a larger filter. I would suggest that you contact your local amateur astronomy club and talk with some amateur telescope makers. Cutting a small disk out of a filter is done in a drill press with a hole saw (cookie cutter!) made from a piece of brass tubing with slightly larger ID than the disk you would like and a slurry of silicone carbide and water. The technique is similar to that used to make an diagonal secondary mirror for a Newtonian telescope, and is described in telescope making books.
