Originally Posted by SteinAir
Many of the modern avionics and EFISes have their own cooling fans in or on them, and in addition to that perhaps the most important thing is to have some sort of vent holes in the glareshield (or even a couple muffin fans). That should allow that dead space between the main panel and sub panel to be ventilated and kept at a good temperature, so everything should be good (as Bob noted, those glareshield fans also work as defrosters too).
Just my 2 cents as usual!
This is what I did. The 6A glareshield isn't' big enough to put a fan in it, so I used two 80mm computer fans in the area behind the panel. One sits near the bottom of the panel, pushing air up and the other sits in the foreward bulkhead in the cutout for the avionics, behind the panel, pushing air into the foreward area. I figured this would keep the air behind the panel from getting stale and hot.