I started a github repo for 3d print source files to attach things to Actobotics structure. First up is a mount to secure a rotary encoder directly to some channel as shown below.
As noted in the readme, I found a few issues post print, so had to switch to subtractive modelling (dremel time) to complete it. I leant a few things from this though so the issues are less likely to bite me in the future. There are some comments in the scad file so hopefully it will be useful to others and maybe one day I'll get a PR to update it. The README file in the repo will probably be the one true source for update info and standing issues with each scad file.
You can just see the bolt in below the rotary encoder. That has a little inset in the 3d part to stop it from free spinning when you screw in the 6-32 nut from below the channel.