I was out of t-nuts and square nuts and I needed these clips to add lights to my printer. I decided to try my hand at designing the clip with the nut/bolt built in and printed in place. This is the result... Based off of peaberry's nice "COB LED Strip Mount for 2020 Extrusions".
You have to remove the built in supports with a knife, BE CAREFUL! I cut myself twice :X
Always cut away from yourself and body parts, it will help to secure the parts in a vise for cutting/removing the supports. If the bolt will not rotate inside the extrusion be careful not to force it, you can shear the bolt in two. Use a file or pocket knife if necessary to clean up the bearing surface of the t-nut.