I have an old HorizonT camera (with T at the end, no plastic, all metal). I wished to make IR photographs with it.
I bought a Cokin IR filter. These are easy to cut in pieces. I have the camera not here, but if memory serves I cut an piece of 17x15 mm˛. At the very edges oft this piece of filter I glued two very small particles of Tesa power strips, the one for sticking posters at the wall. There exist another, not removable version for gluing hooks at the wall, please don't take this.
It is not very hard to put this piece of filter in the slit and stick it in front of the lens. Slightly pressing the filter with the blunt side of a pencil and it is ready. May be you need for removing the filter a needle.