What about instead of cutting, fold a few inches of the top end back over itself. This would give you a built-in air pillow for your head and cut down on functional length. I have seen this discussed, the only person who said they actually tried this said it worked. The horizontal baffles on this pad would seem to make a fold easier than vertical baffles would. Might need to duct tape the fold permanently in place to get it to hold well, otherwise could try bungee or paracord.
Not something I've done, just an out-of-the-box suggestion.