At least here in the US, this square bar seems to be pretty standard in many showers. Mine measures about 20mm x20mm. I created this hook to be printed on it side and snap onto the bar at an appropriate angle to hold your poof/loofah, waterproof bluetooth speaker, whatever takes up precious shelf space that you'd like to clear up without adding unreliable suction cup hooks to the tiles on the wall.

The ones I printed slide a wee bit, so it you want them to stay put a bit more, play around with maybe 99 or 98% scale. :)