I made a few foam seats years back and a lot of people were just using hand tools like rasps, sureform, rough sand paper etc.
I experimented and found that a stiff wire brush on my electric drill worked great. i had a simple 3" very stiff one in a drawer, something like this.

https://www.lowes.com/pd/Kobalt-3-in-Wire-Wheel-Brush/3195521

It will actual plow out a lot of material.

I used a belt sander with 60 grit to form gentle curves on outsides and bottom