I use a old igloo, around 2 gal, that i put warm or hot water depending on B/W or color film,around three to four in of water, then i put my dev, etc in there and put the lid on it, i don't tighten it just placed it on the top.
and I have a thermometer in each plastic bottle and take out each when there close to there temps. works great and the igloo keep its temps for a while. and only takes a few min to heat every thing up.