in my holidays I worked on a easy diy mattebox design with two filterstages. They are not rotateable (that would make the design too complicated for beginners). Now I finished the design to 90%.
I will put the plans as pdf on my site in the course of next week under tutorials section - of course for free!
The whole mattebox can be made of 5mm plywood (recommended) or also of thick cardboard. The two filterframes can hold standard 72mm filters like Skylight, ND, Polarizing... just push the 72mm filter into the filterframe.