Thanks for the ideas.
1. I'll be leaving the back closed except for the holes I cut for the exhaust fan and PSU. My goal is to quiet the system, so that extra little layer back there will be useful for applying dampening material. It'll be easy to baffle those smaller exhaust vents too.
2. Removing or opening up the door is not an option because that would defeat the purpose of using the cabinet.....which I got to help silence my computer when I'm writing or mixing.
The left side of the cabinet is fair game for adding fans or what not, but there's only a 1" clearance all around the case, so the air from such a fan would mostly just hit the case and spread around inside the cabinet instead of being routed to the intake which, for the BQE, because of the downward vents in the bezel, is directly infront and below the case. I still think the idea of an external fan of some sort is a good idea though.