You may consider wrapping a layer of tape around the outside of the shower head in order to secure it fully to the diverter valve.
How to install diverter on outside shower with siding.
Installing a shower system diverter is the key to creating your shower faucet with handheld and showerhead.
Place the original shower head into the diverter and place it tightly into position.
Diverter tub spout with attached shower head.
To avoid siding problems locate the shower away from the house.
You can also find a few options on amazon here.
This article will show the installation of a concealed shower valve with a 3 way diverter as part of a typical bathroom installation in leeds by ukbathroomguru.
You will need to screw this down too so that there are no leaks.
Introduction the installation of this valve allows the user to control the water to their shower bath with one simple control.
If the faucet was mounted at the end of the tub it would have been easy to install a diverter tub spout with a connected shower head like this one.
You may consider wrapping a layer of tape around the outside of the shower head in order to secure it fully to the diverter valve.
Place the original shower head into the diverter and place it tightly into position.
If the shower abuts your exterior that siding will get pelted with more water than it was designed to withstand so go with a material like cedar mahogany or teak and add a waterproof membrane underneath to guard against water infiltration.
Install the diverter valve and cap the 3rd port.