A friend of mine has a camper van the he and I built. It has a boot box that he converted into a power station, and he is now upgrading that battery bank. He needed the front opening to be larger to accommodate his new gear, and he asked me for help. There are several ways to do this, however, using a router has some advantages. Notably, corners look cleaner and when using a plunge bit you get a much nicer finished edge. Below is a little tip that will easily allow you to make your cuts in a straight line.

This box will hold his new battery system. The upgrade requires a larger opening. I’ll be using some leftover scraps to make a guide.
The guide is nailed in place using a pin nailer.
The cut is made using a small router, perfect for a small job.
Once the cut is complete the guides are removed and the nails are pulled out. They are small and don’t leave much of a mark. In this case the holes will be left as-is, but they could be filled if the situation called for it.
A little hand sanding completes this easy project.