If it was me, and I didn't have one already, I'd spend a few bucks on the HF Multi tool.
Use the circular shaped blade, mark it with a piece of tape to monitor the depth, then change over to one of the straight blades to finish the corners.
Probably less than thirty bucks with a discount coupon...
Or, if you're feeling froggy, an old fashioned back-saw and some elbow grease....
