If you use an overhead projector, it can be tough to trace very thin lines, if you don't focus it exactly. You might be better off just enlargening it on a photocopier on ledger size paper to the size you want and tracing it onto your case.

If you do use a projector, create registration marks in the corners - draw four X'es, one in each corner, in a perfect square. Then, when you project it onto your case, make sure that the projected image X'es are in a perfect square - if the top is wider than the bottom, you're getting vertical distortion - if the right side is wider than the left, you're getting horizontal distortion.

Food for thought.