hemm... i know one thing that can be good for this.
in my countri, Indonesia, it's called 'busa hati'
just like sponge, but harder n thinner.
there are several size for the width; 5 mm, 2 mm, or 1 mm
for you, I suggest use the 5 mm.... just like my friends did. they use this 'busa hati' to make tokusatsu costume, keroro helmet, armor, etc...
sorry I just can help you bit like this...