Razor works. My suggestion to get a nice, clean hole with smooth, round edges is to score the shell, then tear it. It will tear at the score line you put in it.

[youtube]http://www.youtube.com/watch?v=fascLqFVtf4[/youtube]


As far as a temp gun goes...I don't use one at all. Three drops of water on the glow plug, if they boil instantly it's too hot, if they don't boil at all it's too cold, simmer over 2-5 seconds is just right.