My subwoofer :D

[The Brick]

Any extra bracing wouldn't fit and I seriously doubt that panel will move enough to have any measurable effect at all. It's 22mm MDF, glued to every side and braced at the back. The front that could possibly move a picometer even has holes in it so it doesn't move air.

 

[Parrot of doom]

Trust me, it will move. Put your fingers on the front surface of the baffle when in use, you'll feel the movement although that will be more to do with vibration being transmitted directly from the cone to the baffle.

I'm not critisising mate, I think you've done an excellent job, I'm just suggesting ways in which you might improve it, avenues to pursue etc.