I have been trying to find one but they have never had stock so I made a plan with some stuff that was lying around the house.
The thought came to me while watching the rain water flowing out of the gutter one evening.
It was very simple and it works very well as all the Toro needs to not water is an open circuit across the rain sensor connection that it has.
Step 1: What You Need
1 x brazing rod or hard wire
1 x lever switch that will give you the option of "off" when pressed
1 x small spring
1 x piece of wood
1 x some glue, epoxy worked for me
Step 2: Start Putting It Together
I stuck the rest of brazing rod onto the lever of the switch with some help of some copper to hold it together while the epoxy dried
I then bent the rod around the tin so that it fitted under the little edge the tin has got. I clamped and glued the rod to the tin with the epoxy.
I started by screwing the adjustment lever into the wood with a small screw and leaving space below for switch to mount
I then placed the switch so that there was adjustment between the lever and the spring to adjust the level of pressure of the water that the tin would hold and activate the switch
Step 3: Almost There
I tested the whole think with some water and realized that once the water fills the tin is has to go somewhere otherwise you will not have any watering done until it's evaporated.
I made a small hole in the middle and the back so that the water would slowly trickle out and the switch would reset
Step 4: Fitting It Next to the Gutter
Notice the holes, do this before you fit it.
It works well, and has been operational for a few months, I sprayed the switch with Mr Mechanic so that it would not corrode quickly and do that ever y now and then just to avoid fitting a new switch.
I hope this helps someone, and gives someone pleasure tinkering in the garage like it gives me.