If it's cutting for grading, what would probably be best is removing the box bottom from the box by cutting partly up the box side. Making a bit of a tray. The sides then may fold down flat, I don't have an 85 donruss box handy to see if it would. Either let the leftover sides down flat, or use a new blade in a utility knife to slice the corners so the excess folds flat.
Once you're at that point use a paper cutter to remove the sides, and to cut the panel the way you want.
Mine is a cheaper version like this one.
http://www.dickblick.com/products/da...paper-trimmer/
It can be tough holding the piece straight for the first cut.
There are better machines, but they're a bit pricy.
I haven't used mine for any cards. My collection of cards on boxes ranges from full boxes, most folded and bagged, to singles hacked out with a dull jackknife (My own from a scout campout in 77 where they had a bunch of Hostess stuff donated and I got quite a few of the boxes. ) But I have used it a lot to cut up cereal boxes to make envelope stiffeners for shipping stamps or corrugated cardboard to ship cards.
Steve B