I've discovered a way to be able to sit for longer (without triggering my PTSD panic button) - it turns out to be music. I put on Steve Reich's Music For 18 Musicians quietly in the background, which has been a friend since the 70's, and the familiarity of it (coupled with its beautiful constant changingness) acts as a protector, and I can rest in myself, indwell, be still and loving. The music in me rises up to meet it...
This is not "traditional", but then tradition doesn't have much useful advice on PTSD! I'm content.