The big advantage of this DIY design over the factory designs, if I see it right, is that this one puts the media on the pressure side of the pump and the factory units put the media on the suction side. That alone makes the DIY vastly superior even if it doesn't save much money. I'd really like to see a diagram of how it's laid out.
I'm planning to try an external filter myself on our next tank. So thanks for posting this. It's encouraging to see people thinking for themselves rather than just thumbing through a catalog.