Here are a couple links for you. There are a couple ways to do it.

http://www.rcuniverse.com/forum/m_17...tm.htm#1771683

http://www.rcuniverse.com/forum/m_24...tm.htm#2478885

http://www.rcuniverse.com/forum/m_38...tm.htm#3822957

10mm compression fittings work the best, but you cannot find them in the states. I used 3/8 compression fittings but they are very tight fit. You have to sand some material off the inside of the fitting until it will slide over the diff cup. Then CA glue it into place. The above links have some pictures of the process and the finished product.