Thanks for all the help. These are all good ideas and have given me an idea of my own. Those items are difficult for me to get plus the chassis I am trying to balance don't have dedicated holes for this purpose. It does however have screw holes along the centre line for things like the bumpers so I think I'm going to remove the countersunk screws and put in some caphead screws and see if I can balance them between the screw heads. If the COG is somewhere between the 4mm width of these screw heads, that's close enough for me
Thanks again.