Beyond the Open Door (Vintage Paperback) by Andrew Lansdown

Did you say DRAGONS?

Who would have believed that a hole in a card-board box could turn into a doorway-from another world? Colyn isn't really sure how or why, but he thinks it might have something to do with the weird knife he used to make the hole. So now all Colyn has to do is put the cut piece of cardboard back into place.

But Colyn can't find the other piece of cardboard. He can't close the doorway.
And the dragons are coming...