re: flex 180 insulation
This won't be quite the answer to your question but you may find it helpful. I use an Exped Downmat 9 inflated to about 2/3 of max with my Flex. Very warm and doesn't slide out from under me despite all the tossing and turning I do throughout the night.
When you're laying in the Flex, a prominent longitudinal ridge forms along the grey fabric panel and prevents the downmat from sliding laterally. The mat may slide down toward the foot of the hammock a bit but you'll still be on it.
The attached photo doesn't have the downmat in it but shows the prominent fabric ridge along my right side.
When I owned an NX250, the same downmat would easily slide out laterally from under me during the night