I believe that they make special devices for stretching the plantar fascia - you can find them, for example, in running catalogues.

I had a very persistent case of plantar fasciitis back in the eighties (I was in my 30's.) My podiatrist finally performed a plantar fasciotomy, and that did the trick. Many might consider this approach too radical, plus the fact that it worked for me, on that occasion, doesn't necessarily mean that it will work for others.

Good luck!

Don