I used a cheap but accurate turkey frying thermometer. I did the drilled bolt mod and it can slide in and out for charcoal basket removal. Works great. In the first pic I pulled it out a little to show slide through bolt. The second pick shows how it sits when smoking.
You need one with a probe that extends at least to the middle of the grate. I have a turkey fryer one on the bottom grate and Ashcroft industrial style i got off ebay on the top one.
When looking at thermometers grab a handfull and pick the one that does not read higher or lower then the others. Check in ice water for 32* and boiling water for proper temp. At my altitude and barometric pressure its about 208*.