I'm thinking 3/4" with good window bracing is all I need. Good point about the front panel. Once you cut for a flush mount, there's not much left to secure the driver.

I was surfing last night and saw some examples of extreme DIY cabinets. One guy built his speaker with three layers of 1/2" MDF. The middle layer was more of a spacer to allow him to fill the core with sand.

thanks,
Gar.