I use a saucepan and a sugar thermometer. You are only cooking at low oil temperatures so there is no danger of it igniting. Just make sure your hob has nice big sturdy stands on it (assuming it is gas).
Hmm, it's terribly disrespectful to disagree with a moderator, but (IMVHO) hot oil and lit gas don't mix, so even if you
are cooking at low oil temperatures, if the oil overflows and reaches the gas flame, I would not be surprised if it were to catch fire. Play safe, never let it overflow !
(I speak from bitter experience : I was cooking crispy and aromatic duck in Canada, in front of guests; on lowering the duck into a saucepan containing hot oil, the oil overflowed and the flames nearly reached the ceiling. Never since then have I used a saucepan in which to heat more than a very small amount of oil, far less than 1/3 of its capacity).