Short answer: any should be fine.
I've just noticed that you say the 11 foot thing- remember that the quilt only has to cover where you are- so the dimension is really how tall you are and whether or not you want your feet covered. (and also remember you lie on a diagonal for the flat lay.) If you want hammock covering goodness, check out the Speer PeaPod, now manufactured by TreeToTrail gear.
Lots of UQ (underquilts) should fit most gathered end hammocks- they are usually designed to fit most hammocks.
You've got a lot of options- down or synthetic, baffle type (how the down/notdownfluff is grouped), 3/4 length or full, suspension system, COST, wearable (multi use).
There is always the Poncho Liner UQ (PLUQ- I pronounce plook, it's more fun that way) and other DIY options.
TH (the only quilts I have are JRB ones, and they are great- but I haven't compared to any other manufacturer)