My son has access to a laser cutter at the university where he works and made me aware of this site http://www.makercase.com/ when I asked him to cut some 1/4" plywood for a case I want to make. You just enter the dimensions of the case and it generates the laser cutter case plans. You don't even have to calculate exterior dimensions. If you know the interior dimensions and the thickness of the material, it calculates the exterior dimensions for you. It gives you a choice of flat joints, finger joints and t slot joints and generates those for you as well. It's a neat web site.
Out of curiosity for those of you who have made cases, what thickness of material do you use? I'm wondering if I could drop to 3/16" luan since the finger joints make a pretty secure join and it does those joints for not only the sides but the top and bottom as well. Or even use 1/8" masonite??? I was planning to add a strip of wood inside the rim so that the screws for the hinges (and handle) would have something more to bite into.