What Tinker said.

For info, I have had no trouble with fillers in bottleneck cases. Just use your noggin and don't load wad material that could compress solid (like CoW!).
My favourite is saddle-felt, but dacron obviously works OK too. In my loads for the 20bore/.577 and the .577/.500 No.2, two 1/2-inch felt wads are needed. I work a fat wad through the neck onto the powder, and a neck-size wad on top of that under the projectile. Also did the two-wad trick with backer-rod on a friend's .577/.500 Fraser double. No dramas whatsoever.