i think a good way to go about it would be to make it rectangle w/ a grosgrain attachment tab on each of the four corners.
then add a channel for a draw cord across each end.
since the quilt is already made, it may be more practical to add a draw cord channel made from some other material folded over.
that way attaching it to the hammock by the grosgrain loops & getting it properly adjusted for the weighted hammock is one step.
the draw cord is a separate adjustment for closing the ends to make it warmer or loosing them to vent.
on the head hole, do you plan to add omni tape to close it?
i'm thinking you will want to find a way add enough length to one of the omni tapes to allow them to over lap with out pulling the quilt out of it's normal flat shape.
i have been planning to add a head hole to one of my summer weight speer quilts too, but i plan on adding a collar instead of the simple slit.