Hey, that’s a clever setup for your Snapmaker A350 enclosure! Skipping the wall hole by routing exhaust under the garage door with a flattened 1.5" x 12" outlet (matching 4" circle area) plus 2x4s for sealing/backflow protection and a bug screen is smart—keeps fumes out safely without major mods. The 4" upsizing for standard ducting/fans and inline booster fan makes total sense for better flow, even if the stock hole is 3". Great for laser smoke in a garage/gym space.
Your design’s actually shared on the Snapmaker forum and Thingiverse (by SteveW25561) as “Portable laser/printer exhaust vent - for use under garage door”—folks have printed and used similar flattened vents successfully. If you’re tweaking it, test airflow with the door closed to confirm no restrictions, but matching cross-sections like you did usually works fine without much loss.
Solid DIY solution—props for thinking it through! If sharing the STL or need tweaks, the community would love it.