what can I use to cushion my knee when exercising with it on the floor so it doesn't hurt?

yes, a cushion, but every one I've found is too big and doesn't fir my backpack where I have more stuff (clothes, an umbrella...).

I like doing yoga and stretching outdoors, so the ground may also have loose dirt, broken pebbles and grass...

If I bend on my fours, raise-stretch a leg with the other leg's knee touching the ground to hold most of my weight with this leg's foot stretched to the back and still touching the ground, raise-stretching one arm and only using the other arm not to fall to the ground, the knee touching the ground hurts.

I've tried folding a small towel three times, but I'm so skinny that my knee still hurts.

Folding a yoga mat doesn't help either.

What works for you?