I really do not know if there is a thinner sabot then the Harvester Crushed Rib. You might try going from say a .430 bullet to a .429 like a Speer Gold Dot, this might reduce the overall thickness of the combination.

On Modern Muzzleloader Forum they have a post on all of this...

http://www.modernmuzzleloader.com/phpBB/viewtopic.php?t=9798

this might help. Also if you can find Sabotloader, he might be able to help as well. I am guessing your combination is too tight? Yes, they show going to a small projectile will slightly reduce the over all thickness..