I use a Paterson tank and tempered water bath (in a small portable cooler). I keep the water bath at about 118F, give or take a few degrees. I pour the developer (at about 101F) into the tank and immediately put the tank into the water bath. The level of the water bath should be about the same as the full tank, so that it won't float. Anyway, I do twirl stick agitation for the first 30 seconds, then put the lid on the tank. I invert every fifteen seconds -- quickly. That is, the tank comes out and gets two very quick inversions (with about a quarter turn between) before going back into the water bath. This goes on for the duration of the developing time. Every fourth inversion stop or so, I might end with a quick tap to release any air bubbles on the film (as in black and white), though things happen so fast with C-41 developing, I'm not sure how necessary this is.

118F might sound hot, but with inversions taking between two and three seconds, the tank is out of the water bath about 20% of the time. In any case, it's after regular and repeated test runs (with water) that I came up with this process. I'm measureing the temperature in the middle and at the end of the developing time. This is with 500ml of developer and an ambient temperature of about 68F.