Those vent holes look like you cut them with a standard hole saw drill bit...am i correct? heres an idea that I used many moons ago, when i built this gigantic burm, boa, whatever the hell else i could fit in there...go to radio shack....get some speaker covers..they sell 4"-12" metal circles with bunch of holes in them, the type you see on the ceiling. OR they also sell plastic square(with rounded corners) that are quite nice looking and they dont cost much at all...with the plastic you can mount the bracket to the wood, then snap the plastic cover on to it....they work quite well..