You just need one of these.....
http://www.myoldtools.com/boxjoint/box1.jpg
http://www.myoldtools.com/boxjoint/box3.jpg
http://www.myoldtools.com/boxjoint/box4.jpg
http://www.myoldtools.com/boxjoint/box5.jpg
http://www.myoldtools.com/boxjoint/box7.jpg
It's a purpose made box joint machine to cut multiple sides at a time. The carriage moves back and forth across the multiple cutter head. The head can be set up in 1/8" increments. Built in the early 20th century, prior to ball bearings. It has been changed over to ball bearing now, but i have the original babbit bearing boxes with it. If you want to go into the business, let me know
http://www.myoldtools.com/boxjoint/box2.jpg