Don't know exactly what size you need, but I use a multi-output flat wrench. Here's what I mean:
General purpose:
http://cgi.ebay.com/TM-Black-HC-Flar...QQcmdZViewItem
From the jato kit (rightmost tool):
http://cgi.ebay.com/NEW-TRAXXAS-JATO...QQcmdZViewItem