Pretty sure you're not finding a sheet of aluminum large enough for an undertray at HD or Lowes. A sheet of aluminum large enough for an undertray is probably going to cost you atleast $100-$150. Once you factor in you time and the tools needed for sheet metal fabrication you're easily into the cost of the Z Speed undertray.
Do yourself a favor, buy the Z Speed undertray and save yourself the frustration of trying to make a quality one yourself.
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'11 370Z Nismo #528 Fast Intentions | Stillen | Motordyne | Uprev | CSF | Zspeed | Z1 Motorsports | AST | Swift | SPC | SPL | Whiteline | Hotchkis | Bride | Schroth | Robispec | Cusco | Nismo | Volk | Forgestar | Hoosier | RJM
'17 Titan Bone Stock
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