We used this kind of an idea...
http://www.chiefdelphi.com/media/photos/36149
http://www.chiefdelphi.com/forums/sh...threadid=89991
By putting two tubes on the minibot, and then two pins on the robot, we never had any problems with it falling off or coming loose. we used about 4 inch pins and the shorter tubes to save weight.
It does great with side to side movement and keeps the minibot in a perfect vertical orientation.