I was on a job site a year ago or so and saw this groove cutter:

I think what the carpenter did was he got a longer bolt for the saw arbor and just used standard saw blades, then offset them so the teeth went in between the teeth of the adjacent blades.
It seemed like a simple solution to making a wide blade cutter.
I don't know all the fine details but it shouldn't be hard to do if you can find the right thread bolt for your standard skill saw.
Jim Rogers