If you install something with an STC (Supplemental Type Certificate), you're using that documentation as the acceptable data, and a Form 337 would be required. Even though it's a minor alteration, the approving data effectively changes the type design.
However, if you don't have a 337, that, of all things, is not a deal breaker. Simply a small paperwork issue that your mechanic can handle very easily (and inexpensively). Easiest way to get past that is to order the club belly drain kit. It comes with all the paperwork necessary to file a new 337. If I was your IA, I'd just file it as a new modification and be done with it.
