i'd go with steel, 1/8" plate would be fine for what your doing, keeps some of the weight of the suspension. i would bring some 1/2" thick strap steel around 2" wide from your factory bumper mounting holes on out, and build the rest off of them, just put any tow hooks on the end of the strap steel. and a thicker (1/4" - 1/2") brace going across the whole thing wouldn't hurt anything.

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its a ATV All Terrain Volkswagen