My very first solution was nails in the wall and hanging them on those. This works if a, you don't mind nails in your wall and b, if you have a limited number of rulers.
Then I found this plate draining rack. Which was OK but again limited the number of rulers.
Then there was this solution, an Ikea table top with a small shelf under the glass.
So the search was on for a simple inexpensive solution and I found this little plate rack. It works and for about £5 it is a bargain.
If you click on the picture it should take you to Amazon UK's page where they sell it.
The ultimate solution would be this clear custom made cutting mat and then go back to storing them under the glass. I shall have to wait until Monday to find out how eye wateringly expensive it will be.