Attaches to a standard 12mm pipe, which is very common in Europe. In America this is ' 0.4724'' ' or 'almost 15/32 inch (= almost 1/2 inch)'.
The Fusion 360 file is included and you can edit the pipe diameter there. Even beginners can do it easily. Fusion 360 is free. There's a standard "FM Mattsson" shower holder (only the part that goes around the hand shaft) in the Fusion 360 history too if you want that.
The holder can rotate around the 12mm pipe some, about a total of 90 degrees. It can also move up/down the pipe with ease, just keep fiddling with the screw tension.
Print the volume around the screw and some of the shaft leading up to the screw with ~75-100% infill, to make it durable (had it break once). The entire back piece should be ~75-100% infill too.
And that's it! I made it so there's a clearance between the screw shaft and the hole it fits into, but you may need a fairly well calibrated printer. If the shaft turns out too thick, just sandpaper or file it some. It'll go fast.