If I use one 15"x13" brace with 4"x3" cutouts, and run 1/2" x 1/2" vertical strips up each corner, with 2 horizontal strips per panel, I get a volume about 5 ft^3 less than what I calculate for an unbraced cabinet built with 5/8" stock. What effect will this difference have on the woofer alignment? Would this be sufficient bracing to compensate for the thinner walls?

For additional vibration-killing, what about gluing some asphalt roofing tiles to the insides of the panels? It shouldn't affect the volume much. Or you could use a marble plinth and thinner marble top, "sandwiching" the speaker between them, or...

The mind runs amok.

Tanks for all the help,
--Shannon