Needed something to hold a Altec Lansing Mini Life Jacket Bluetooth speaker in the shower to keep it clean and from falling on my feet. I came up with this shelf that hooks over the top of the shower wall and supports the speaker out of the way.
My shower wall is about 24mm and it holds it perfectly

Printed vertical with supports under the open shelf sections(might not have been needed)

0.12 layer height

10% infill

Hatchbox Transparent Blue PLA