wool batts are very lightweight---but lofty--super nice- the dream wool would probably be a good choice in your situation--it is washable- very nice to work with= and inside the sandwich probably will not bother you---remember however- you will have to handle it to get it into the sandwich in the first place.
if you want heavy a good double sided flannel will add weight- also you can layer a cotton and a wool batt together---the cotton would add the weight-the wool the warmth and loft.
fleece is a very warm-heavy fabric and can be used for batting too.
you may want to contact a company that will send you swatches of batts so you could check it out before commiting---wool batts are on the expensive side- it would be sad to spend alot to find you can not use it-
try the dream company- they may actually have a swatch book available for all of their fabulous battings!