Internal Braces:
That oblong looking thing on the left is a circle cutting jib I made. The router bolts to the wide end with the cutting bit going through the hole. There's an adjustable pin that goes into the hole's center. I like it because it's infinitely adjustable.
For the bottom brace, I drilled holes to accomodate threaded inserts which would screw into the base. I wanted the base to be removal should I ever need to get into the crossover.
I made a little jig to help with inserting the threaded inserts:
And the finished product:
Heres the bottom of the base where the screws thread into the bottom brace: