Right now I'm using the stock plastic cup. I just discovered these:
http://cgi.ebay.com/ws/eBayISAPI.dll...MEWA:IT&ih=014
So I may order one of those and see if it helps.
I haven't tried stuffing a GP Savage diff into the housing, but I'm willing to bet large amounts of money that it won't fit - a Maxx diff is too big, and the GP Savage one looks even bigger from the pictures that I've seen.