I experience the same problem you do when I don't velcro the Nest to the HH slit. Velcro'ing the Nest to the HH solves most of the problem for me, though - I don't get drafts through the slit like some people.
It must be different body types that lay in the HH differently that causes the underquilt to fit a bit differently for some people.
The only thing I can really think of to keep the slit closed is to hang the foot end a bit higher, so your body slide up towards the head end and puts less force on the opening. Maybe lay a bit less diagonally and see if it stays closed. Then if you can get comfortable that way you'll be good.
Let us know how it works!
“Republics are created by the virtue, public spirit, and intelligence of the citizens. They fall when the wise are banished from the public councils because they dare to be honest, and the profligate are rewarded because they flatter the people, in order to betray them.” ~Judge Joseph Story
- My site: http://www.tothewoods.net/
- Designer, Jeff's Gear Hammock / Pack Cover
IMPOSSIBLE JUST TAKES LONGER