I started with a turkey-fryer thermometer hung through the lid, making sure that the end of the thermo didn't bump against the meat. After I got my first ET-73, I just dangled the probe through the lid vent just to the curve of the probe. Keep in mind that the temp in the lid where a Polder-type probe measures will run 5-10 degrees warmer than the top grate (usually). Might check your probe in boiling water (should read 212) and in a slurry of crushed ice with water poured over it (should read right at 32-33). You can make sure that you're reading accurate temps that way. You can also poke the probe of a polder-style thermo through a chunk of potato and set the potato on the grate next to the meat, running the wire under the edge of the lid. You can set the lid down on it and it won't hurt it - just be gentle.
Keri C, still smokin' on Tulsa Time