I have a little gadget that was at one time made by Nikon. It holds a 3 x3 filter and is on a funny spring ring set up. It had barn doors, so I took those off and with wide angle lens clamp the filter holder on the back of the lens and place the whole piece back into the camera. Works just fine.
You might check Freestyle. at one time they had a filter holder that may do the trick.
Sorry to be so vague about the description. You just push on the spring part to establish the correct size, drop in the filter and shoot.