It’s a really simple 337 that your local IA can do. No field approval needed. I have a PA-18 project I am doing for myself. When I sold my PA-12 I kept the TOST hitch which I intend to put on the 18. AC 43.13-2B chapter 8 covers it pretty well. https://www.faa.gov/documentLibrary/...2043.13-2B.pdf