I'd use the existing end plates as spacers, cut 2 new identical end plates. The existing end plates will need cutting down in size to allow the new plates when fixed to slide correctly into place.
The new end plates, as well as being seam welded around the edge should have a couple of holes drilled in them to allow the centre areas to be welded to the existing plates also for maximum strengt.
I'm self taught so please don't just take my word for it, but thats the way I would solve it.