Do not know if you have a shop or steel yard locally but maybe call around. Someone may have something already cut out to your specified size Or at least close. Our stuff just gets delivered out of yard in Birmingham so no experience with any of the online vendors. Looks like you can order a...
I am not a reloader but I work in a machine shop so I do know a little about rust. I would say steel but would suggest spraying it with a very good primer or even a “bake on“ type enamel if you think rusting will be a problem.