I’m not sure if this one has been invented before but it’s a pretty good stretch I’ve been doing in the shower.
You grab palm up behind the head with your whole fist, then point your dick upwards and kind of turn your hand as if you were pouring something out of a jug. So your little finger works as a fulcrum because you’re pulling down with the top part of your fist and up with the bottom part. While using your whole fist to create a strong grip. While stretching do a kegel for extra effect. As it’s going sideways you’ll have to do an equal amount for both sides.