This is not my original idea, but it sure is a great
one. I got it from an art magazine years ago and
only wish I could acknowledge the clever artist
who shared it.
These are called 'eye hooks'. They are a great
solution for caps that have disappeared. But better
still, they are a great solution for the way the necks
of paint tubes get all loaded with paint and it is
impossible to screw on the caps because the screw
threads are full of dried paint.
You will notice that there are different sized screws.
Different brands of paint have different sized holes.
So experiment by buying several sizes. Even if the
size you have doesn't fill the hole completely, it will
keep a channel open for you to get the paint out
I have found that I almost never bought a size big
enough when I started doing this years ago.
The artist who wrote about this did this with all
his paint tubes and then hung them from something
he made so he could easily find his paints. I never
did that. Just doing this was a great help.