I agree with the fan. The interior of the AVR can get pretty hot, pretty fast. Wish I did something similar since had a glass divider two inches above the top of the AVR.
My Onkyo's HDMI board died on me a few months ago, and thinking this could be one cause (on top of subpar quality boards they used on some production runs, as confirmed by Onkyo some months ago). Would cost me 15k to replace, so opting to use my HDMI Front with an HDMI switch.