[Humabout]

Honestly, you probably don't want a sabot if you can avoid contact with the rails in a railgun; you could just make the rails helical and use a maglev-type configuration to float the projectile in the weapon. Current could be transmitted by plasma (this is one layout, minus the helical rails, that has been proposed for railguns).

For coil guns, you can just use the magnetic field itself to impart a spin on the projectile. This, again, is probably more efficient than a sabot because you are not losing a ton of energy to friction and you don't have to contend with the heat buildup in the gun, either. After all, circuits generally don't like heat.

More efficient than either of these is using a fin-stabilized projectile, or if the round is large enough, an actively-stabilized and possibly guided projectile. If you don't like fins, a lot of current research is being done in the use of piezoelectric and memory-materials to create deformable control surfaces for everything from entry vehicle heat shields to airplane wings that deform instead of having control surfaces. This could be extended to the surface of a projectile, if miniaturized (which, frankly, is probably easier than making an airliner's wing deform)

With all of that said, this is ultra-tech, so you can really just make up whatever material you want. Some bioferrous smoothium-balongite nanocomposite discarded sabot totally works if you need to justify it!