WAAAYYY back in the day I built one using a flat, the bends were pretty crappy done in a vise but it functioned. Requires welding on the handgrip/magwell tube and the rear of the receiver. Main reason I did it was I picked up a bbled upper with bolt for about $20 at a gunshow. These days a sten mag tube is available and is a better option, $3-7 steel mags versus $20 plastics with weak feed lips.
The flat you link to is for a MAC10, not an M11-nine. Two different guns, not compatible with each other. The only $84 item I see on that second website is a bolt, not a complete kit. Complete kits run about as much as the complete guns sell for, so no savings rolling your own. The website also looks suspiciously like Florida Gun Works, who are famous for not having anything in stock but will charge your card in an instant.