many companies sell locking systems for engines, some go in the glow plug hole
http://www2.towerhobbies.com/cgi-bin...?&I=LXD567&P=7
, but the better idea would be to get a special wrench that will hold the flywheel like this one:
http://www2.towerhobbies.com/cgi-bin...?&I=LXGFK8&P=7
the last one is much easier on your engine.... i would go with that one