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Here are some pics of one I just cut out of a piece of scrap aluminum I had here at the house. Remember I have no shear or brake at the house so I had to cut it with snips and bend it over my work bench so no smartelick remarks. j/k This should give you an idea of what I was talking about. I think it would look awesome made out of diamond plate!
If it was 12 1/2" or maybe 13" you would be able to go right to it. Come up from underneath with the self-tappers. Might have to have an extension for your cordless drill but you could get that at your local hardware store for a couple bucks. Drill your self-tappers out by the ends then move in as far as you can and put a couple more.