I found that plain ole Nerf darts (the foamy little things our rug rats still manage to break things with) will fly out of my Cold Steel big bore with pretty good velocity and it has decent mass, knocking empty Coke cans over and sticking to non pouros surfaces quite aggressively.. I'm still in the planning phase, but I think I will test to see if I can sharpen a golf tee, slice a Nerf dart into thirds and try to make my first sabot round.
I use a sabot for my homemade air cannon when firing bb's. I just melt a little cavity that I can fill up with 30 or so bbs and then let 'er rip! I don't know if there would be much of a point in using a sabot because of the increased surface area on the projectile, but for firing multiple darts or even bbs it could be fun!
Sabots are also useful for firing sub caliber projectiles such as a 3/8" steel ball out of a .625 barrel. A good sabot for this is two orange cones on either end of a short piece of dowel with a magnet on the outside of the barrel to prevent the ball from rolling out.