Well my room box finally dried but as you can see from the picture below it had suffered badly from cracking during the drying process. Because it is in the corners I think it must have happened because of trapped air.
I used the remaining papier mache to fill the cracks. I covered them in some thick PVA, pushing it into the cracks then I filled them with the papier mache. I was really careful to push it in firmly to minimise the amount of trapped air. Then I smoothed it all out.
I will have to wait till it dries...again....to see if it has worked.