Super Strong Magnets
A stick to mix the epoxy together with
Now just put that magnet, and 4 more in the corners and let it dry overnight.
While that was drying, since I had most of the supplies out, I finally turned the geological markers from our trip to South Dakota into magnets too.
I just used our Dremel to cut off the pokey part then using the same epoxy glued the magnets to the backs.