width... i wouldn't skimp to much on the width myself. just the other night i was reminded about cold shoulders, elbows, but, & knees<g>.
shape... i think "canoe" fits the shape i like best... sorta like the hammock it' self. except a canoe w/ both ends cut off.
with most under quilts there's extra stuff at both ends that is gathered up to match the smaller ends of the hammock, but unless you want the extra width & drawstring so you can vent, that's an area you can shave weight & bulk.
one thing i have thought about adding to my next underquilt is a thin, down filled, draft collar on the inside of both ends to create a good seal.
attachment... i like the idea of sewing loops onto the hammock edges.