They are a little more expensive, but probably the easiest way is to stick the Hot Hands Adhesive Body Warmers in there. The tacky side will keep it from sliding around. I have zippered mesh pouches in my Arctic Shield ones that I sewed inside by hand and then tied a pull rope on the zipper. That seems easier with gloves on than working a button. I guess it's no big deal to sew neoprene and I could put a big button and big loop, these are sewn at the seams of course....but they are expensive and I've never sewed neoprene, so I'm a hesitant.
PS...figured out a work around...sew the zipper pouches to the existing mesh pouch in the TN ones.