You could go to home depot and get some 1/4 inch nylon spacers to put in between the top and the chassis. You may have to get some longer screws. This would stop most dust from getting in and still let out the heat.
ARC Ref 6 plexiglass cover
My brand new ARC Ref 6 black was delivered yesterday 6-15-18 from ARC and to my big surprise was the twice as thick plexiglass cover that came with it. I had bought a used Ref 6 silver,natural finish that i had problems with it from the shipment (came damaged ) any way the year that the used one was built was 2016 and had the twice as thin plexiglass cover which seemed to melt on top of the power supply tube. It looks like ARC corrected this problem and now uses the twice as thick plexiglass cover , i can not see this cover melting and causing it to bubble on top of the power supply tube. Nice improvement ARC !!! And i will tell you that right out of the box it sounds AMAZING..