Personally, given the boxy style of the M41-A Pulse Rifle, I'd prefer the look of a box magazine over that of a drum style magazine.
There are a few box magazine kits out there for airsoft. None that I have seen are meant for the Thompson, but could be altered to do so. From what I understand of airsoft box magazines, some of them actually just use the internals from a hi-capacity magazine, and the space within them that are used to contain the BBs are opened up so that BBs from within the box magazine can continually pour into it.
Shouldn't be impossible to construct one ourselves.
-Alex
"The M41-A. When you absolutely, positively have to kill every mofo in the room. Accept no substitutes."