O.k. so maybe "I'm going to write a section of this pieces as a double cannon at the 5th by inversion" was making things hard on myself. Especially since I really didn't want it coming in all "And Now: RENAISSANCE ERA COUNTERPOINT!!", which is a big danger when you're doing modal composition.
BUT, I knew that I'm using a lot of root movements by 3rds. Which works out really nicely in inversion at the 5th.
Plus I don't care about approaching and resolving dissonances in any sort of systematic way. Which simplifies things so much.
Mostly I just wanted to flex that I wrote a short double cannon at the 5th by inversion. And it sounds stylistically like the rest of the piece. Just a denser texture.