This is a kit that has a 1/4 turn valve to be placed between your shower head and the neck from the wall as shown in the picture. The kit also has a thick rubber disk that you place inside your tub spout to block water from the source pipe. That makes the water from your shower valve flow up to the shower head all the time.
You will turn the water on with the shower head valve open so you can adjust the temperature once. Then use the shower head valve to turn the water off and on. The temperature you set then never changes.

NOTE THE PRICE INCLUDES TAX AND SHIPPING.