A home made tool can be fabricated from a piece of board and a hole saw. Select a saw of a size similar to the size of the filter ring (slightly larger is better of course). Drill out a hole near the edge of the board (or drill it in the centre of the baord and then use a circular saw to split the board thru the middle of the just drilled hole). Keep the centre plug of wood from the hole. Take the board and mount it so the dge with the hole is facing upwards in a vice. If you place the filter ring in the "half hole", dent side down, you can then use the centre part of the hole that you have retained to press down (or tap down) so it presses on the inside of the filter ring - you may need to file the plug a bit smaller to fit inside the filter ring.