I would not go near 8 ply rag with hand cutters. I have not heard of anyone who can do a truly professional job with anything other than a computer controlled auto gizmo...and that is after time spent setting up etc.
I have a Keencut ultimat and the blade would not protrude far enough for such board . In any case there would be a lot of bend in the blade and the quality of the cut would be dire. I cannot stand poorly cut mounts and cutting 4 ply to the highest standards requires care, even with good kit.