This is the second version of the foot rest I created. I wanted something a bit more substantial than the source thing to keep my 2 year old's feet from getting stuck between the tire and fork. It worked okay with the foot rest mounted above the frame (https://www.thingiverse.com/thing:4384235) but didn't give him much space so I created this version with the foot rest mounted under the frame.

I used two of these adapters, each requiring the following hardware: two 3" long (2.5" would probably also work) pan head stainless steel screws and 2 nylock stainless steel nuts, all in 1/4-20. M6 hardware should work without modifications (things will just be a little loose until assembled).

I printed these in ABS at 70% infill after sizing the parts to 102% to account for shrinkage.