I use a simple Rubbermade container water bath AND a 1 minute pre-soak. I pre-heat the developer and blix to 39C and also prepare a pre-soak (same volume as developer) at 39C. This way the tank is already close to 39C before the developer goes in. Using a Paterson plastic tank with the pre-soak and keeping it in the water bath except while inverting I don't lose temperature over the 3:30. The blix is not as crucial, can be even warmer like 40.5C as it goes to completion. Rinse water again isn't as crucial, 35-40C or so. Don't let it get too hot though, you can melt the emulsion at 50C.