Eating the box won’t hurt them, you’d be surprised (and horrified) by what they can eat without doing themselves damage! Highly coloured things with lots of printer ink should be avoided though.
I agree with the other comments that a rearrange might switch things up a bit. Guineas need a bit of space for a mad moment. Maybe a more square shaped box tipped upside down with holes cut through it so they can hide/run through it. Guineas love hiding. Old toilet rolls stuffed with hay are fun (for them at least!) just give them different things to explore and do and they’ll love you forever (well, between that and food of course!)
Best wishes with your new pals!