Kapok was one of the original filler materials. It's a natural fiber (from an eponymous tree) that was used to stuff life preserver vests back in the day. Shooters adopted it as a case filler. Today, true kapok fiber is no longer used as floatation, and is pretty rare to see. Synthetic fibers like Dacron batting take the place of kapok as a very fluffy, almost weightless filler. Dacron allegedly burns completely in the cartridge. I have always had lingering doubts about it depositing tough plastic residue in the bore, even if it works to keep the powder near the primer despite jiggling and vibration (which I doubt).
I don't use fillers, either. I'll select a bulky powder, or a very easy to light powder and simply shoot it as is. Such loads aren't for target use, so I don't worry about supposed velocity variations.