In earlier days these splinter mats were made of rolled up hammocks but of course that's not what you're looking for.
Definitely NOT sandbags, as that wouldn't work in a vertical plane.
I have one reference that calls them splinter 'matresses' , but another one mentions that 'splinter mats were welded to the bridge' which would mean at least the outer shell was metal?
Then again someone mentions that they were also used as makeshift life rafts.