If you are using plastic comb binding, this file may save you some time cooking up custom covers. It has worked fine for me, though it is possible that if you double it to full letter size the last holes may creep out of the ideal spot–I would definitely do a test on paper before committing an expensive material.
I just bought a Cinch. Not quite comb binding, but same basic idea. I originally wanted a GF to cut my bookboard. I still haven’t bound a single book. And now I’m going to “Cheat” with wire binding!