Polar scope - O-ring to stop small optics wobble inside

It's been a year since I decided to buy a scope and begin to explore the heavens. I must admit that it was a great year, got pretty far with gaining experience using the scope equipped with a dslr and how to capture different objects with different imaging methods. Eventually I had my first photos of planets, meteor, Moon and ISS. So I love my Skywatcher Explorer 200PDS+EQ-5 mount.
But still there is one thing I couldn't sort out yet. Polar Alignment... Slowly becoming my nightmare, tried many many different ways, watched tutorials etc.
One thing became soon clear, if I do everything the right way than there must be something wrong with my equipment's settings. I had an attempt to remove the polar scope, but only the bottom part came off (small piece of optics with Cassiopeia and Big Dipper on it). After that whenever I turned the RA axis around whilst looking through polar scope the little optics in the polar scope was wobbling, making polar alignment impossible.
Later I've been told they put a rubber O-ring in between the optics and the small tube, as optics inserted the O-ring keeps it right at a good fix position.
Of course it was probably one little step to resolve the misery of my polar alignment issue, but this might be useful for someone.
My big question was the size of the rubber ring. I did my researches on that but found no answer on exact parameters.
Well went into a DIY store, bought 5 different size of O-rings I thought might fit the requirements and luckily one did. I had to acknowledge the wobble has stopped.
Hopefully one problem less...

Inside diameter 18.0mm
Section               3.0mm

 

 







This video I found and is a perfect explanation of this problem. By the way many good videos can be found on youtube under name Astronomyshed, same gentlemen who's in this video too.
From 2:30 becomes interesting.......