Remember 2x4, 2x2 1x3 ect isn't really spec, it's less. I'm sure if the cab is smaller, less bracing is in order. Real 2x4 = 1 3/4 x 3 3/4. Regardless, I like to use a 2x4 T brace at the top of the woofer across the baffle to the back wall of the cab. Where the leg of the T hits the back, I put an approx 12" square piece of 3/4" ply to spread the load on the back panel. This really stiffens up the cabinet.