Kiln dried sand.
Buy some, spread it on a baking tay and bake for a an hour at 70Deg c for an hour.
Sieve.
Then fill.
You want the top of the bag to give a bit and the leather to slide over the sand.
Give and flex.
Before or after the fire?I did that too as the sand was wet after being in the garden
After the fireBefore or after the fire?