I've had showers in over 50 airports across the Americas, Europe , Asia and elsewhere in the world.

The airline lounges' locked shower rooms mostly have had enough space for me to store a small wheeled suitcase and a backpack/laptop bag on top of the wheeled suitcase while I take a shower. Otherwise, there's been a workable alternative to secure my belongings.

The airports that have showers outside of the airline lounge/paid airport lounges/in-airport-hotels don't all provide soap and towels. But the lounge showers seem to mostly all have towels and soap provided.