I mounted mine on the rear upper plate(G-maxx chassis), right next to the motor(G-maxx motor mount). I used a couple layers of foam to pad the bottom and zip-tied it to the plate. I then routed the battery wires around the tranny towards the front to plug in the batteries. I also zip tied the battery leads to the x-brace chassis braces to prevent them from touching the spur gear.
I found the perfect place!!!! It dosnt need a fan because it stays cool. It is out of the way and everything. If you want I can make you a custom bracket to mount it. I charge $500 per custom bracket.
This is actually how I mounted mine. It is bolted on to the old motor mounts. You should be able to find something close at a hardware store. Some kind of L bracket.