Protruding wall heater

Does anybody have a wall heater in their listening room? Mine protrudes out 10" and is 40" tall. The upper area where the vent is located is where the mirror lands for first and second reflections using the sound reflection mirror test.

When sound hits that heater area, there’s a slightly annoying treble haze if you will. How to treat this spot? Obviously, I can’t cover the vent openings. I keep picturing some kind of portable panel I can put in front of heater for listening sesh’s and take it away when not listening. I can’t find any such products, with the exception of a bulky tripod contraption on Amazon.

Any suggestions?

Thanks,
Paulo