Give your designs away and ask for donations. You have a small group that even believes in magic, and creating new runes is hard work. Presumably, a large portion of that community values your work and would support it voluntarily. So don't even attempt to restrict copies. Instead, make your work available for free and ask for donations so you can continue your work. You could also ask people to donate to your development of a particular rune and then make it available once you've raised enough cash.
There's tons of precedent for this model working. Many browser extensions and apps are free and then ask for a donation. Lots of people raise money for a purpose by providing a service and then asking for donations (car washes by students, bake sales for charities, charity bundles of Steam where you name your price). Kickstarter and Gofundme are both full of people that voluntarily give money to support a cause they believe in.
So don't waste a bunch of effort protecting your work. If you're providing a valuable service, then ask furfor donations, and people will likely pay more than you'd have charged of their own free will.
