I've tried balling a coat up and my exo as well. I feel like there has to be a better solution99% Of the time I am in the field with my Exo pack. Inside, I strategically place a down coat that works well as a rear bag rest most of the time, as long as I am not wearing it. I practice this way quite a bit and have used it out to about 670 yards on elk. It is obviously not as solid and as versatile as a nice rear bag. The rigid pack frame also works well as a rest if it is positioned vertically (for sitting close range shots) if needed.
I am tagging in to see what works for others as I have often asked myself the same question.