Quite a few designs are already approved. If what you want is in the dropdown list, you don’t need much for documentation. I form 1’d a DB9-22 and just put in the description that it was the same lower as the DB9 Alloy (already in the approved list) but with a 22LR upper receiver and barrel. I included a rendering of the lower with an arrow showing where I would put the registration info and that was good enough for a 10 day approval.