I was just wondering..can I recycle cardboard boxes for the cover actually?
Or do I need to have a bit more harder cardboard?
I usually use some sort of paperboard. A thicker posterboard works well, and so do cereal boxes. Anything thinner and the cover is a bit flimsy. Anything thicker and the "hinge" gets too bulky to open well. Don't use corrugated cardboard -- way too thick.
As far as recycling goes, along with cereal boxes, check out cracker, cookie, rice and dry soup boxes, as well as old notebook covers and notepad backs. I've even used some thick junk mail as covers!
Alternately, you could even try a thin plastic (a thin plexiglass maybe), but I'm not sure how the gluing would hold. Haven't tried it yet.