Looking further at the screens, there appear to be two choices:
The Adafruit Capacitive touch screen:
https://www.adafruit.com/product/1983
This is about the size of the PI and attaches to the GPIO, but uses different pins to the OpenSprinkler.
Or the Pimoroni/Official touch screen which is 6in (ie. homebrew mini tablet sized). This attaches to the “Display” connector on the Model 2 & 3.
I’m leaning towards the cheaper Adafruit display.
Either way, I don’t think the OpenSprinkler box cover would fit, so I’m thinking of mounting all components in a wall-mounted box with a latching door.