I've made two so far.
Basically, when you use your rune, you apply any effects as normal, but also you test luck. If you make the test, you keep the rune, if you fail the test, your rune is gone. Each time you pass the test, put one determination token on the rune's card. subtract 1 from your luck stat for each determination token on the rune card the next time you use the rune and roll to see if you keep it.
When you use a rune, draw another rune from the remaining runes and keep it, then put your previous rune in the stack of rune cards, and finally shuffle the rune cards. Repeat each time a player uses a rune.