[happy camper x2]

Same problem with our 2018 353 RL. Dealer installed 2 more returns added two new discharge ports. They matched the factory ones. Also used 1 1/2 pvc pipe to open up return and discharge plenums. We measured the distance of pvc needed cut to length and used a cap when pushing into place then removed the cap so no flow blockage. Wood might snag the insulation.

Actually works like it should.
Good Luck