You just need one of these.....
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:)