Evan,
They do have some internal bracing between the side walls and the top and back of the cabinet. I wouldn't say they are extensively braced compared to what I have done for some home subs in the past. I could always add more bracing if I needed to.
As far as building a modified version of these, I am sure you would be happy with them (a dozen of them that is
). Building the flared port was the only mildly difficult part.
As far as measuring them I may take you up on that. I will PM you. Would love to hear your new Mackie HD's as well. I have been wishing our church would buy a pair of those 1531's to go on top of these for the youth services.
Thanks,
Loren