Even with super think material, with a moment arm that long, it seems like it will bend if you put something in the vice and start cranking on it. Did you think about adding any sort of vertical brace on the top of your mounting arm? I know you couldn't put anything under it because it wouldn't pivot back onto your bumper, but you should have room to put a 1 inch piece of something vertical most of the length on the top. It would really stiffen it up and let you do almost anything without fear of bending the mounting arm.