My RV has Furrion Chill 14.5kbtu air conditioner units and the air volume coming out of the air distribution unit is very high, and the little gates included with the distribution unit cause lots of wind noise. So I took off the cover and found the hole where the air comes out of the fan and into the air distribution unit. I measured the hole and designed a sliding gate valve that goes in from the top (stick it through the hole and pull it back so the neck fits the hole). The sliding part may need some deburring to fit easily, but you need some friction in the gate to keep it where you want it to stay. If you close it off completely, all air will go through the ducts (if your RV has them) with no air coming out the front and rear of the air distribution unit.

I printed mine on a Bambu Lab X1C with black PETG

Even with the heater element installed, PETG will be stable.

Let me know if you have issues with other AC units.