I put a group 31 box on, came with 2 clips that the strap goes through then box sits on top. I screwed them down which was a pain!
If you do screw any thing down drill pilot holes for screws, then drill a bigger hole through top angle iron piece only. This will help aid in not breaking any screws as it will only be attaching to the bottom square iron.
You will see rust coming out where the top angle iron and square tube meet, run a bead of some black silicone along all that, i can see it rusting away in time!
15' Coachmen apex 288bhs
10' crossroads zinger (sold)
09 rockwood roo 21ss (sold)
08 dodgeram current TV