To use those as underquilts you would PROBABLY be best served by sewing cord channels into each side, so you can run some shockcord through there and rig it to the head and foot ends of the hammock.

However, to play around with it, you can use an old quick-and-dirty tarp trick.

Place a smooth pebble on a spot on the bag where you want to attach a rope, then gather fabric up around the pebble, and tie a rope around the whole bundle. That'll let you attach ropes wherever you want, and play with the position you're putting the rope.

Making a DIY underquilt will require some patience, as you'll basically have to just mess around with it a lot. If you've got trees in the backyard where you can hang the hammock to tinker, it'll help a lot.

My gut tells me the poncho liner won't be enough insulation, but you won't know until you try it - in most cases, you'll want your underinsulation to be AT LEAST as warm as your top insulation.