Bruce, Sarco used to have a nice reproduction 100 rd Drum pouch for the Thompson. It looked much like the military 50 rd pouch. They haven't stocked them in years though. I got mine from them almost 10 yrs ago, here's a photo of one.
Iv'e also used a Chinese, (I think) drum pouch that has more than enough room for the Thompson 100 rd drum. Don't recall where I got it from. I don't know why Scarco stopped carrying the pouch it was very nice. The Lewis drum pouch is kind of a snug fit for the C-drum. Anyway let us know what you come up with.
David, feel free to use the photo of this reproduction C pouch in the pinned thread on pouches.