There was going to be a strip of magnetic tape at the bottom of the groove. I bought a roll and touched the end of a stamp to it but the hold was so negligible so I abandoned that idea.Is there a magnet inside the groove?
This has all 36 including the numbers.That may take a little more space but has certain other advantages.
I may build a couple for my sets. If I made it two stamps wide I could include the numbers also.