I've been struggling with the same problem here --- not helped by the fact that my darkroom, such that it is, is in the Attic -- turns into a hot torture chamber by the first of July. I recently started developing film down in the basement (helps a bit). I also use *frozen* plastic bottles of water to equilibrate my water bath-- it's important to circulate the water when you do this, I have used a relatively large picnic cooler + aquarium pump with styrofoam floats/cutouts to hold the chems.
My husband recently took pity on me a found a used Jobo TBE to help maintain the temp. Looking forward to trying that out.
JT