Will your grate go in the slot upside down? If so, that would eliminate the raised edge at the back. It would be angling down. I wonder why oven manufacturers make grates like that anyway. It seems totally pointless. What purpose does a raised back edge like that serve? A flat grate would be fine. My oven has weird raised-edge grates like that too.
You can go to any steel supplier in your area and have them cut a rectangle of steel for you of any dimensions you want. It'll be much (much) cheaper than buying one of the steels from bakingsteel.com. Although it might not be as pretty.
Some people shy away from 1/2 inch thick steel because of the weight, but with your shallow oven depth, I don't think the weight of a 1/2 inch thick steel for your oven would be too heavy for most people. Make it a couple inches wider than it is deep, and you'll have an easier time landing the pizza on it.
Measure your oven carefully to determine the maximum depth you can handle. I'd probably ignore that raised edge at the back and just plan on resting the steel plate on top of it. Then level the front of the steel by putting a length of square tubing of appropriate size under it. Maybe 3/4 x 3/4 square tubing? Or some 1x1 angle? Whatever will give you a more-or-less horizontal surface. It doesn't have to be perfectly horizontal.