Trimmed starboard plenum bottom, floxed it to the top and clamped with cleco.
Finally figured out that it was the remnants of the port cowling rib that was misleading me about plenum clearance. Removed the remnants around the plenum, and checked clearance starboard. Taped the inside of the upper cowl around the plenum hole and the plenum exterior and hot glued them together with wood blocks and sticks and various random floor debris.
Prep sanded inside and patched with 3 x UND 90/45 with one ply between crossways.
Had some resin left over, so taped the inside joins 2 x UND 90/45.