I've seen tons of those. they are cool, but really annoying. It actually seems really simple to make, you probably only gotta fab an adapter to go into the bottom of the blender. the weed eater motors have teh little output shaft right there so its nothing complicated, cool though.
Makes me remember in HS when me and my friends tried to build a weed eater powered BMX bike. only thing we had a prob with was we didn't know anyone to weld what we needed, we had all the parts though