I'm assuming # 10 screw size being used here for your grounding lug/hardware. First, install the grounding pan head screw thru the opposite side, then install a Star Washer under the first nut (or Hex Keps nut will suffice), and tighten firmly. Now, your grounding lug(s) can mount to the top of this nut. I'd actually use a flat washer, then another star washer on top of that, the lugs, then lock washer and nut. Keps nuts to secure the lugs tend to spin the lugs, whereas separate nut/washer don't have that tendency. For long term protection of the now-exposed bare metal, once the grounding screw has been installed, you could apply some clear lacquer over the expose metal (not over the grounding screw hardware).