If you have a really good hardware store near you....the kind that has all of the little plastic cases with slide-out drawers and all sorts of little pieces and parts...then look for metal (usually chrome) or black plastic plugs and grommets. They're so cheap this way, that you can usually buy a bunch and not worry about size, and go home and try them all. I like the black plastic ones, because you can paint then to match your firewall (if you insert them from that side), or you can dab some silicon on them to keep them in place and never come out. Of course, if you're doing major work to the body, just mark all the superflous ones, and weld them closed.
