I just used a t-bone racing servo guard.. doesn't cover the hole but will block most debris from getting that far and also covers the servo so crashing down on stuff doesn't risk that getting hammered.. It's made of black nylon instead of polycarb (don't get the clear one if you get one, it cracks too easily)
am sure you can google 't-bone racing slash 4x4 servo guard' on your own so I won't bother posting a link...
those aren't all that great because rocks and stuff can get in behind the servo guard and actually cause damage to the arms. This is one of the first things I looked in the buying for my truck because I didn't like the way the servo arm was exposed like it was, but after hearing the complaints after asking about them, I changed my mind. Are they as bad as people say?
I did just order the Tekno center drive shaft and saw on the Traxxas forum someone who used heatshrink along the whole shaft, which is what I may do. I did remove the clear plastic cover as well, debris gets in there but it falls out easy as well.