That's why I have an insulated water bath. Turn it on 1.5 hours later it's at temp. From that point on it barely runs. Even with it turned off it holds temp fairly well. I've turned it off at 98F and the next day it's still over 90F. This isn't outdoors during the winter but in a 60F basement. The other thing I know I could improve things. Right now the lid can't even close with the wires in the way. If I really wanted to improve things drill a few holes route the wires and fill the holes in with silicone. The thing would hold temp even better.
OTOH once you get past the critical step [developer with C-41, 2nd developer with E-6??] the temp range is much wider. More then 5F I think.