Are you using a resevoir or an inline system? I was a chef once upon a time and had a food thermometer lying around. So I just drillined a little hole in the top of my res and used that. Works pretty good, except that the plastic was melted while in use in the kitchen.
Close-up:
Whole Res:
I would imagine that if you have an inline system you could rig up something similar with you're airtrap or "T" or something. You can get these thermometers anywhere they sell kitchen products (pot, pans, etc.) for about $10 (US).