Flow Restrictor for Shower

On my quest to live sustainable I realised that I don't want to miss out my hot shower in the morning. So I built this flow restrictor that fits into the screw connector part of my shower.
The upper outer diameter is 10mm and the lower outer diameter is kind of 7mm.
I only found flat inlets here on thingiverse so I thought I could design something that doesn't render the black rubber seal useless.

- It has a perfect fit: You can press it in by hand and remove it still with your leatherman