Good luck welding/tacking that sheet metal. The "box" area on my backsplash was too big and the carriage hit it which limited travel by about 1/2" or so closest to the chuck. It was also going to be tighter than i'd like once i mounted the DRO scale so i trimmed an inch or so off from it and capped it. I'm talking about a sheet metal cap parallel and closest to the ways. I'm not sure what the metal is but it didn't want to weld. I made it work but i've been careful not to post pics of that embarrassing mess.
I think it'll hold fine and it is sealed up enough to keep chips away from the motor but it just looks like crap. I can only assume that they back splashes are somewhat universal and that's why the motor box area i so much larger than it needs to be. Your motor fills that area much more than mine - i wonder if they kept the same design even after they switched to variable speed DC motors that are smaller?
The spot welded strips look nice. A little green hammer paint and no one would be the wiser.