I found a few designs to convert the Ferm rotary tool to a router. Unfortunately, they all turned out not very stable when I built them. I wanted something that would not move when I am using it, especially since there would be force applied and vibrations when trying to cut wood.
There are four parts. I designed room for bolts and nuts to keep everything sturdy, turned out it is not needed as it is well kept in place by the snapping mechanism. I left them for those who would feel like they may need it.
The square insert is kept in place using the clamp part. It may be tricky to remove (just pushing on the other side with a screwdriver does the trick) but it keeps the tool well in place, it should not move at all!
The rest is pretty classic, the insert slides into the main part. It can be snapped to a base with additional mounting holes if one want to connect it to a jig or whatever.
If you do not use the optional screws for additional sturdiness (as I do), the only additional material you need is a pair of M6 nut and bolt to keep the slider at the desired height.
The pictures are from a slightly different previous version, no dramatic difference.
Edit it on Onshape: https://cad.onshape.com/documents/003adee0007ee72e6f1d41af/w/d67da886e024d8db27fb4ade/e/dfbcd9f72019a30207da90a7?renderMode=0&uiState=669227479a0787550305b6ef
The author marked this model as their own original creation.