Hi - I have done what you are trying to do - I'll assume that the 2" on the thermometer refers to the probe rather than the dial. It should work without the coupler. I purchased the plug that is hollowed out as I was afraid of the same thing. I wound up putting together what is shown below over time as I transitioned to quick connect fittings. For me it works fine and isn't much harder to clean than I think using a solid plug would be.

2" probe and plug works.

I used a Blichman 2" thermometer and the hollow plug to create a inline thermometer on a stainless counterflow chiller. Has a ball valve and a cam lock. Works like a champ!

you will need extra length.