Some thing I have used as sabot/wadding is "stress ball".
You can get them typically in 2.5 inch diameter but they will easily compress to smaller barrel diameters.
They are made in a closed cell foam which works better than open cell foam.
Obviously for combustion you would want some way of flame proofing.
Here on the end of the barrel is what they typically look like.
Closed cell foam makes poor squish balls.
To squeeze closed cell to 1/2 volume, it requires at least 1 ATM of compression pressure. Foam pool noodles are closed cell. Most squish balls are not. Try squeezing a pool noodle to 1/2 size.
To tell if it is open cell, squish one. Let go. Does it retain it's compressed shape for a moment (memory foam), If so, it's open cell and the air returns slowly.