I have a fair number of old lenses with no threads or weird sizes, so I often tape filters behind the lens with gaffer's tape. On a couple of lensboards, where the lenses don't protrude too far, I've epoxied 3x3" filter holders to the board, so I can also slip a square lenscap on when the lens is not in use. This is particularly handy for ultrawides, where even a step-up ring for an oversized filter on the front threads will cause vignetting.