Malgray Chipboard A4+

A4+ sizing: 250mm x 337mm - It is a little longer and wider than an A4, which is 210mm x 297mm.

Uncoated recycled greyboard. High-quality, flat grey chipboard. Eco-friendly, sustainable, recyclable. General bulk and stiffener applications. Care to be exercised to control moisture to ensure flatness.
These are thick cardboard pieces that make up the covers of your notebooks, but you don’t often see them because they are usually wrapped under a paper or cloth layer.

BUT, these are not acid-free.  If you don't mind your books not lasting about 200 years then you may safely use these.  Acid-free generally increases the longevity of books.

1x A4+ Board