there's another approach, all though most people seem to like more sag to get the diagonal like angrysparrow said.
i use an insulated air mat too & i usually hang my hammock pretty tight. but i l only partly inflate the air mat, allowing it to conform to my body (sleeping on back or side).
w/ the tighter hammock & soft air mat, i get a fairly level lay.
i also use a pillow of some kind (clothes in a stuff sack) between my knees on my side, or beneath my knees on my back.
and a neck pillow for... well, my neck<g>.
as for the sliding, raising the foot end higher on the tree a few more inches should cure that.
hope it goes well. ...tim
don`t leave the CREATOR out of the creation!