Started with a whole beanbag, because I could form it to support my lower back and legs. But after some time I realized I didn't need lower back support, so I used a cheap old couch cushion instead.
The cheap old couch cushion turned into a pancake after a few weeks, because the stuffing was inferior. I turned a few couch cushions into pancakes, then looked around and settled on a Salubrion chair, which offered exactly the right tilt for my body form. But I still had to put a small cushion on top of it, because otherwise it was too hard on my butt. I used my pancakes for that.
Since then I've settled on a nice round cushion filled with natural ingredients. Works fine. I'll usually shove a wadded towel or thin blanket or something under my right leg, because the knee still hovers above floor-level, although the left knee touches down. I find that anything that elevates my butt about 2-3 inches off the floor is enough to allow my back to achieve a natural straight posture. I can do straight on the floor, but it's not really ideal for my body form.