No reason to spend $100 either.
Most people don't live particularly far from an operation that sells metal. I was gonna say urban, and then i recalled that your more agrarian regions fabricate things out of metal too.
So I'm gonna say, if you don't have to drive 50 miles to buy groceries, you probably have somewhere nearby that sells all manner of steel.
And they may have a scrap that they can trim down to fit your oven. 1/4" thick is ok, 5/16 or 3/8 is better, 1/2 is not necessary.
I bought a 14x14x1/2" slab for $40 including tax and cleaned it up a bit. I'd have rather bought a couple 15x10x3/8 slabs but they didn't have any.