Anneal the metal before each step. Making sure that the center post is the right height to support the crest where you want it, bend the bottom end to form the foot or tongue, and grind or hammer it flat to fit the holder on your helmet. Finally, file, sand, and polish it well. The arms can be squeezed together slightly at the top to give them a firm grip on the crest block. Crest supports made of brass are apparently more common finds than iron, but quarter-inch square steel stock is readily available and easier to work.
Many depictions show a comparatively short block mounted directly on the helmet.
Coolus helmet crest knobs often have a slot crossed by a hole. The crest block would be quite short in this case. Montefortino helmets generally have a knob with a vertical hole. This is acceptable for Legion use as long as it is red. While you will probably want to display your crest when not in use, it is better to store it carefully.