http://www.friscorodgun.com/aap/
http://www.alpacamania.com/alexsbait/html/page9.html
Or, if you have a good local welder, he can custum make one for your rig.
Have to keep the steel painted, so, the aluminum, might be the way to go, also it is a lot lighter.
I have steel racks front and rear on my motorhome, paint them once a year, but they were a deal, and that's what made my decision!