I've built this one twice - neither time with any particualr dimensions or plans - just kind of threw it together as I went along.
It's fairly self-explanatory - it throws disks of cardboard traced from the bottom of a 5 gal bucket and duct-taped together in 3 or 4 layers.
To be honest though, we usually end up just tossing the disks by hand - its easier than reloading every time.
It is nice if you want to shoot by yourself though (just loop the trigger rope around your ankle to fire it)