I used the box section and made a lug to weld to the end and the mounting bolt welded to that. A lot of hack-sawing. Then I welded the bolt onto the plate, then welded that to the end of the mounting tube. It will be plenty strong enough as most of the weight is carried by the bottom mount.
The gap will be filled with a strip of lighter sheet metal, and I plan to do some detailing to help with the looks of the fenders.