Is it not easier to but some plate and make your own box ?
Cut it out.... weld a new bit in job done.
I bought a YRM one, so recently that I'm still in the process of fitting it! It fits very well, and is easy enough to do - the hardest part was removing the rusted nuts to take off the sill rail, and the fact that my little rivet gun isn't man enough to pop the stainless rivets that come with it. (My knackered hand doesn't help either........ surgery in 3 days!)
Did you look at other alternatives?
How far do you go with it tho . Can't you get galv ones that rivet in ?