Automatic Water Shutoff - Tight Spot

I have to install an automatic water shutoff in my daughter's house. Previous owner had installed shower in room with water main.

We have a 12" wide 2' deep access to main.

Shower is to the left. Notice T for shower cold water. Outside wall is to the left. Main is about 1" off of back wall. Room behind is a finished bathroom with vanity/sink on wall.

My 4-year old grandson can access. If I empty my pockets, such in my gut, and turn sideways, I can slip in. Only can use one hand.

Any suggestions on how to attack this would be welcome.

You should ONLY need one hand. What do you mean by "automatic" and why do you THINK you need it?

Insurance company

In that case you have to decide WHAT operates the "automatic shut off". Is it water on floor activating sensors, in which case you need a solenoid valve and its controller. IF "constant flow", or "sudden flow" then you need the appropriate device in the line ahead of the valve. ALL of these will be "difficult" to install in THAT location because of the space limitations.