I couldn't easily find a good illustration link, but if you go to grainger.com and look up their item # 2P236 it will be a representation of what you would use. Now 1) the Grainger item is a national brand (Parker) which is overkill for what you are doing, 2) the price is for a package of 5, approximately four more than you would want, probably, and 3) for one item like that, the handling charges will probably be more than the $3--$4 that you will pay at a local hardware or plumbing outlet. Tradiitional automotive parts stores also have these. Don't worry too much about the pipe thread size; you can use regular fittings to accommodate whatever you want to plumb the thermometer into.
Under that hex nut at the left side of the illustration would be a ring-shaped ferrule that slips over the shaft of the tube (or, in this case, thermometer) and is compressed when the nut is tightened, making a seal between the fitting body and tube. Most of these fittings have a stop surface to keep the tube from going all the way through; if you don't find one that is bored through, you can easily do this yourself. In the absence of proper tools, run the fitting into a pipe tee, and use a C-clamp to hold the tee at the edge of a bench or table while you drill. Obviously, you don't want to mess up the conical sealing surface, so it might be best to use a slightly oversize drill, entering from the far side and continuing until the stop edge is just gone and the thermometer stem will slip all the way through.
The ferrules are not reusable, but they are sold separately, so if the thermometer dies you just cut the stem, take off the nut, and use a new ferrule to install a replacement.
I can probably find one of these fittings in my artifact collection and take some d*****l snapshots if the foregoing doesn't make it clear enough.