never tried sabots, so these are just thoughts....
I use masking tape (1.5 layers) to hold the charge to the bottom of the ball in my Hern mortar (the charges are really light and it is hard to get them to go in straight any other way). The tape leaves the ball at firing. If you want them to separate, this may be a good way to do it.
If you want them to stay together, how about a drywall screw thru the bottom of the sabot and into the ball?
BTW, what is your ball diameter?