Headphone stretcher / expander

I made this to expand/stretch plastic headbanded headphones. For example the Sennheiser HD560S, which is a great headphone I think, but the clamping force is just too high for me. I've tried to expand with different objects, but I was just struggling to find the right size.

Important: be careful. Do not overstratch,
Do it step by step and insert it for several hours, try on, adjust and do again.

You will need a hex head M5x25 screw, a washer and a wingnut.