I use a Swiss Army Fleece Mummy Bag Liner as part of my insulation system. A lot of the time here in Texas, its all that's required. I sleep in it just like in a sleeping bag. It has the advantage of having a draw string at the top of the bag. I attach that draw string to a prusik knot on the ridgeline:
It may be possible for you to do something similar. It still takes a bit of squirming to get the bag just right---pratice makes perfect---but the bag does not slump to the center and I can position the foot box while sitting on the bag and then swing my legs over and into it without having to do that outside of the hammock.