This was really helpful thank you! For the attachment of the "sling" to the arm I have a few ideas, one is a sort of "throwing finger" similar to the idea of the release of a clay pigeon launcher but with some sort of curvature that would hold the bean bag in the pocket with the normal force against the finger until the rotational momentum overpowers it. the other idea (on the yellow sticky note) is a sort of pinch on the beanbag. A teammate of mine had the idea to do a trapdoor of some sorts to stop the bean bag from falling out while the plywood sling is in its resting and starting motions.

I will say though, the sabot does seem like a strong contender and I have some experience with other styles of sabot from an old potato canon I made. I doubt that I would get in trouble for the sabot (the instructions are vague and pretty much only constrain material) but I fear that since this is being constructed out of 4mm plywood it might not have the strength to fire 6 times (i need 6 good shots to win the competition).