There are 2 options.
Automatic bread knife, fast fun but not that good in details ->
https://www.youtube.com/watch?v=wqodjNexy7w
Melting it, with a hotwire. Slow, toxic fumes, and issues with breaking the wire but it is good at details ->
https://www.youtube.com/watch?v=fi3CAtpvJJs Automatically Appended Next Post: old mattress foam or old chair foam is dirt cheap to come by and works perfectly
Automatically Appended Next Post: Glue always makes it stiff. Construction kit / glue works fine, it does make it a bit less flexible.