My point is that by partitioning the box you spread the load thoroughout the volume. So my point is that you really don't need the double thick box walls if you have enough braces. The added mass just makes your enclosure heavy and hard to move. Now nothing is wrong if you wish to build the ultimate box but just make sure that you have plenty of help moving it into place. I'm just thinking that you are going to an overkill situation vs. a good enough solution.

Your angled corners should work to disperse the load among the structural elements in the box.

Jim