HI dodster,
There's nothing to say that you can't fit any type of temp guage into a Weber ( I think you're referring to the kettle) I've seen huge temp meters in the kettle models that didn't come with a temp guage.....it all depends where you place it.
If you place it at the top of the lid, the temp will be somewhat different to the heat at food grate level but if you place a guage near the lip of the lid, then the probe would be close to the food grate temp. That's of course if you don't have the guage probe too close to where the fire is.
You can buy a kettl temp guage directly from Weber in South Australia
https://www.weber.com/AU/en/home/
or get a different one from Amazon or ebay.
If you're going to drill a hole in the lid, make sure you place some tape over where you're drilling because the enamel paint may splinter if you don't.....it's just to stop the baked on enamel from cracking and splintering when the drill bit goes through it and you drag out the bit again could lift up the enamel off in the process. The tape will hopefully prevent that from happening. I'd place the gauge either next to the top vent on lid or under the top vent between the vent and the lip of the lid.
Where are you located, if it's somewhere that i know, I might be able to direct you further.
Regards
Davo