Sabots are typically used to improve the seal of projectiles in the barrel, be they slightly undersize or much smaller.
The improved seal (or larger area for the gases to push on) improves muzzle velocity, and a smaller projectile flies farther and penetrates better. Foam is the most commonly used sabot material because it is strong enough for most spudding applications and it's light weight means that less energy is wasted accelerating it.
There are two primary kinds of sabots; ones that enclose the projectile and those that merely sit behind it (also called "wadding").
Both are commonly made out of foam, being either insulation foam cut with a muzzle knife or expanding foam cast in a segment of barrel-sized pipe.
After this initial step, the sabots that simply sit behind the projectile are done, but the enclosing sabots need to have the hole cut out and possibly be cut into 2 or more pieces.
Foam sabots don't travel very far, typically about 15 feet, and can easily be found.
Other than that, sabots can be used many times unless sabot collapse, in which the projectile exerts too much force on the sabot (breaking it), occurs.
Such an event can be avoided by putting a hard covering on the sabot, protecting the sabot with padding, or lowering the pressure (and thus forces) used.
• Caps from miniature spray paint cans make an acceptable sabot for a 2" SCH. 40 PVC barrel. A slit may need to be cut down the side for a proper fit.
• Rags or paper towels can be used are are more effective when damp (which reduces the chance of catching fire in a combustion spudgun).