The stock one is inaccurate and prone to warping.

Instead, I put in a New Braunfels thermometer ($30, Home Depot). I got this thermometer a couple of years ago; I notice that Walmart carries a very similar unit for only $20.

The New Braunfels thermometer is very accurate. However, it requires a larger hole to mount than the stock Virco thermometer. To do this I enlarged the hole in the hood from about 1/4" to 7/8" using a bimetal hole saw. IN order to center the hole, I bolted a 1/4" piece of plywood onto the hood using the existing thermometer mounting holes.

The resulting hole was a litte oversize, so I found a large brass washer in my workshop to cover the hole and also the original mounting holes, which were no longer needed. This helped stabilize the mounting and also seal off the gaps. To secure the New Braunfels I found a large nut in the plumbing supply department of the local hardware store, and fastened that to the threads protruding inside the hood. The nuts rests against the inner lining of the hood, which gives a little, but which is good since it acts like a lock washer to help keep the nut in place.

I did a test run with a very accurate glass oven thermometer resting on the warming shelf. With all three main burners going full blast, the oven temp reached 550F in less than 10 minutes. With just the central burner, the grill reaches about 450F. The new thermometer responded much faster than the glass thermometer, but once the temp stabilized they both agreed within 10 degrees.

Here is a link to some photos of the new thermometer and the brass washer backing.