[Lippy]

I think you should put bracing through the middle of the box both horizontally and vertically that would stop the top bottom and sides from flexing and maybe copy the bracing on the back of the box on the front!???

Also have you considered putting some sort of sound dampening material on the insides of the box? either the foam thats shaped like egg boxes or acoustic wadding?

Just a thought as my old auto acoustics stealth box had it in and it made a hell of a difference when I took it out to put in a new box, stopped alot of the resonance of panels and gave cleaner bass!

oh another idea if the back and front of the box are exactly parralel to each other then the sound waves bounce straight back at the subs dunno how you'd change that now though, thats why most pre built boxes have a sloped back or sloped front.