bucket with a spare heater
I just guage the temp by hand. Touch tank water, touch new water, you can tell if it is the same.
I have a bucket that holds most of the water I need for a water change at exactly tank temperature, but I do add some tap water to make the full 50% daily water change as the aging bucket holds 18 gallons, less what the pump misses at the bottom of the bucket, and I need 25 gallons for a water change. Fortunately there is not much difference in pH between the tank and the tap, at least the last time I checked anyhow, that may change come winter.
I worried about the fish getting unaged tap water, but I treat for chlorine as it goes into the nearly empty bucket and they get about 10 gallons of fresh tap with the 15 gallons of aged water. They seem to prefer the larger water changes, tap or not, so since I have been banned from using the 55 gallon drum to store water in the kitchen, this method has to do.