When, after many years of heavy use, I lost one of the nubs, I remembered that I had once seen an alternative on a German bike touring forum. Unfortunately, several different local hardware stores didn't have the required parts. But hey, McMaster-Carr to the rescue!
Here's the before:
And the after:
Much nicer, isn't it?
These are the required parts, from left to right:
For a full conversion of a Back Roller you will need thirteen of each. I got everything in stainless steel, which is probably a bit overkill and quite expensive. If you're paranoid about waterproofness, it might make sense to add a little Seam Grip before tightening.
Credits for coming up with this solution go to GeorgR.