I think I have been watching too many videos by the Jeremy Yoder (The Mad Scientist), but he mentioned more than once the value of Tel-Tru Thermometers for offset smokers. I looked them up and they do make drop in dial thermometers for all types of grills. I found one for my Weber gas grill:
BBQ thermometers are available with 1-3/4 or 2” anodized aluminum dials and are constructed of heavy duty stainless steel & ideal for barbecuing.
www.teltru.com
They are sold out at Tel-Tru but Amazon had it and it is only $28 and so I got it. It was a perfect fit and now I get much better readings from the Weber. I highly recommend this!