Lol, cutting with a box cutter would be an epic fail and you would probably hurt yourself in the process. A jigsaw or dremel would work for straight lines, but if you are doing circles or curved lines, I would use a holesaw. Painting the inside of the case is pretty straightforward. You need to take all components out of your computer, possibly derivet to make the process easier, and tape off anything you don't want to be painted with 3M painters tape. I am doing the same thing and am probably going to use krylon paint. I think I will be using the krylon primer
. You have to sand the metal before and after applying the primer and there are lots of tutorials about that process. I'm going to be buying 220 grit, 360 grit, and 500 wet/dry grit for sanding. You will also need a sanding block if you don't have an electric or pneumatic sander. You have to clean the parts after sanding to remove any dust, dirt, sweat, ect. Get some Wax and Grease remover, lint free paper towels, and tack rags. For protection, wear nitrile gloves and a filter mask. You will probably want some compressed air for quick removal of sand and dust particles before using rags, but that is optional. For the actual painting, if you are deriveting the case, you can hang the parts individually from the garage ceiling for painting or just do it on a work table. Make sure you don't kick up dust when working, as it will go into the paint and you will notice some problems. Make sure you don't paint when its too humid (40-60% is good). Be patient and good luck.