The scale for the pattern above is 1 inch = 1 cube.  Use a photocopier, overhead projector, or paper which can be found at most fabric or sewing notions stores which have 1 inch cubes for scaling up patterns.

If you intend to use this pattern for heavy documents, add 2 inches to the over all width(12 + 2 for letter size, or 15 + 2 for legal sive.)  This will give you a wider opening.