Quote Originally Posted by 36 sedan View Post
You're right, it has no diagonal bracing, didn't pay much attention to it. Add a vertical 2 x 4 between the upper and lower horizontals at the corner 4 x 4s with a 12" square gusset of 1/2" plywood at each corner screwed in place would take care of it.
Attachment 65290
Your design would work fine. The only comment I have is that, with the original and the extra gussets, it would take a whole lot of lumber, and be quite expensive.