He Col!
I love the Polyethylene foam packing material that comes in many electronic shipping boxes. It's designed for cushioning and is not rigid like styrofoam. It's easy to cut, doesn't create millions of loose styrofoam dust pellets when you cut it, is malleable, and stays in place.
😉

Table tennis balls work well I am told by the Buxton model boat club I bought a box of 50 for very little money and glue them in with a hot glue gun.

I have never used any and still have all of them.
I suggest when the model is finished you do the 'Upside down test'. As it says turn the model upside down and see what falls out and then go back and secure it.
Roy