A plastic strap with holes (selecta band) is attached in the shower handle and provides a sturdy hold.
How to make
Unscrew the shower handle and thread in the “selecta” band.
Paid for by
The selecta band is no longer available, but Varsam has a product that can serve the same purpose: an elastic strap with two loops that can be placed on flatware, etc (Strap); see varsam.se (in Swedish), last checked July 2020.
This tip is from 2007