This is the method I would use.Skil saw what the saw lets you access followed by an oscillating multi tool to get into the corners.
You might want to build a jig for the skil saw cuts to help guide the saw in a straight line.
Before cutting I would press on strips of painters tape to guard against top side tear out. Use a new blade. When it comes time for the multitool. Use a utility knife to score the cut line. The multi tool will not give you a clean cut like the saw.