I am relatively new to solar but if it's like a computer the fan running to cool the unit can draw in dust then the build up of dust will hold in the heat and reduce the cooling efficiency of the fans.
Ok, I don't know about the dimensions or how much heat comes off the inverter, but how about an industrial metal switch box, something like an Eaton, as they have sealed doors?
Could it be possible to sandwich the back of the switch box between inverter on the in side and the heat sink out side? This way the box becomes part of the heat sink?
One idea begets another, maybe it can be improved