Honestly - for a bit of effort and very little money I would buy some MDF and make a 'mock' backbox at about one third to half a normal backbox depth.
All cut at the same angles as a real backbox ( make sense? ) I would then mount the BG's in there and hang that on the wall so it looked like a real backbox ( only a lot shallower ) hanging off my wall.

You probably wouldnt have to router out a channel for the glass, if you made it all just a touch bigger than normal you could put a thin rail in front and back to hold it in place ( havent done one yet so I am not 100% positive on that but it should work )

One sheet of MDF about $15.00, can of spraypaint $7.00 , half - one days labour = free.
MUCH better end result.