Toes out and heels down. I worked with longer stirrups while cantering. When I mastered keeping my feet in long stirrups, it helped when I went back to my regular length.
Like Mia and Alannah said, remember heels down, toes up. It's great that u already try to not grip with your knees but also think about wrapping your legs around your horse's barrel. Maybe try to two point or getting into half seat in the canter to really get a feel for getting your heels down. Also relax your ankles because I also realized recently that a lot of tension in your ankles can also prevent your heels from going down.
I agree with Mia. Try pushing your feet into the stirrup more. Easier said than done of course, this happens to me a lot with my horses bouncy canter!
Try shorter stirrups 2-3 holes shorter, also try to press your heel down more into the stirrup. You might want to try some no stirrup work to strengthen your leg! Hope this helps :)