Here's a cheap alternative to the expensive steel-focusers(~100€), took me 1-2hrs, it's a refurbished desk ****)
I think the design is quite self-explanatory, it's not pretty and by no means professional but it helps A LOT when adjusting focus.
Not only getting focus but also maintaining it has been an issue for me.
My lenses focus ring turns so easily, sometimes I think it might be the wind which caused my stars to run out of focus (yeah it probably was temperature haha). So being able to lock it down is awesome.
I'll eventually create a metal version, sadly I currently don't have the necessary tools for that.
Hmu if you have any questions about it, I hope this helps!