I just finished the ventilation system and thought it was time for some new pics.

I found a couple of aircraft vents mis-listed on eBay, for cheap. I replaced the gaskets and felt washers (which were rotted), cleaned up the inner iris ring, and re-painted them.
Then, riveted them to a couple of steel plates and mounted them to the front of the pit's frame.
The fan is a 10" AC fan in a MDF enclosure, with PVC fittings and 2" hose running back to the vents.
The whole thing sits underneath the monitor deck, on the front electronics cabinet.
SIDE VIEW
FRONT VIEW
CLOSE-UP
ORIGINAL VENTS