This is not the right size but it's the same make.
I got some that was 12cm by 17cm.
And it's Hardboard, but best of all....
I can cinch it.
You need to wiggle it a bit to punch but it does go through and won't break the punch.
I did make 3 books but gone now, I used it as covers for my smash books,
Cost was £1.99 from "the works" who always seem to have a sale on.
Have another pack left so think I will make another one just so I can take a photo and post it tomorrow to show and tell.