John - we just us 1/2" pex with a brass plug. We try to have the sensor in before the pour to make sure it is seated in the plug but there are ways of getting it there after. We use a utility box on the other end with a round knock out for the pex to slip into. If you have some scrap 3/4" after your install that would work as well.

I have used 3/8" in the past which works well but 1/2" if far more common.