Rubber and paper/coated gaskets seal better than copper or metal/composite gaskets - the only reason to use the latter two is for high temperatures where the first two would degrade. A paper gasket with high tack spray will work for most things.
Quoting JNR:
Good ideas...
Curious if thin copper would be a better choice?
As for tracing the outline, I'll probably measure it all out for the holes using my trusty venier calipers and not too worried about the perimeter, except that it goes beyond and looks halfway decent. I was going to suggest you can take a thin piece of gasket material (for instance), hold it down steady, and press around the larger holes and/or use the old pencil and tracing paper technique, with 'sketching' on an angle so the holes will come thru. You know, like they do on forensics to see the nasty note you left before.