Traxxas is coming out with a molded plastic "hold down" part in a bit made for the MMM and it will end up being the best solution to this problem. It screws down on the ESC and seems like it will safely hold it through most all crashes.
Losi 8 Buggy + Truggy & Associated SC8
Tekno RC Conversion Kits on all
MMM, MGM, Xcelorin ESC's
Tekno Neu "square can" brushless motors
Polyquest - Neu - Flight Power LiPo's