I printed the source adapter but it was just a little too small. Instead of modifying the Sketchup files, I remade it in OpenSCAD and made it parameterized so that it would be easier to modify in the future. I added a small cutout on the bottom piece for an exposed brake line.

As is, the parameters and included .stl should work for a standard seat and a bicycle with 35mm diameter frame (I have a cheap one from Fred Meyer, a Dynacraft Men's 15S Northern Ridge - https://www.fredmeyer.com/p/dynacraft-men-s-15s-northern-ridge-bicycle-black/0008787620990). I used two of the seat adapters, each requiring the following hardware: one 1" long and two 2.5" long (3" would also work) pan head stainless steel screws and 3 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.

I put everything together and decided that I needed a foot rest of some sort but a bit more substantial than the one in the source thing. I created two different versions... one for mounting the rest on top of the frame and one for mounting on the bottom of the frame:

Top of Frame: https://www.thingiverse.com/thing:4384235

Bottom of Frame: https://www.thingiverse.com/thing:113872