There have been a couple of flat finishes pictured in magazines recently with captions and descriptions of the finish. And they were simply rubbed with what ChuckW mentioned. Only they weren't then sprayed with flat.

It takes a bit of technique, but it works. And lasts.

One of the pictures was of a P47 in the finish we used late in WWII, unpainted aluminum. The builder used regular shiny Monokote but rubbed the shine off. It looked real. When looking at the picture it dawned on me that it would save time, effort and wear and tear on the aircraft to do the rubdown prior to covering the airframe.