But if you are otherwise comfortable with a mid-line lie, you might just put an extra piece of clothing ( if you have any with you) under your knees. Or, put your feet out to the hammock sides, left foot to the far left hammock side and vice versa for the right. For me, this tends to form a bit of a foot pocket for each foot to drop into, plus some fabric hammock support under the knees. And lastly, you might experiment with your position either more or less towards the foot. This often seems to alter my knee extension and my heels ability to drop into a "foot pocket" on each side.
Also, all of the above ( in my Speer or Claytors) varies somewhat with amount of sag. Happy experimenting and tweaking!