If you know a telephone cable splicer or cable repairman, smooth talk him/her out of what we called " encapsulant ". Why ?
It's also called Ballistic Gel. Same stuff !
Once you shoot into it don't throw it away. You can add more to it , fill up the holes and reuse it. For us old folks, it smells just like the old art gum erasures.

I've seen some that was soybean oil-based, and some that was a petroleum product.

Both appear to be flammable, so I'm not sure if you would want to use them as a ballistic medium.