You could still machine a snap ring or e-clip/retaining ring groove on a hex shaft.
You could also try one of those push-on style self locking rings.
Lastly, they sell clamping collars with hex bores which may work.
__________________
-Travis Covington
2008-2017 - Engineering Mentor of
Team 254
2001-2008 - Engineering Mentor of
Team 968
1998-2001 - Mechanical Director/Driver/Member of
Team 115