Use a hole saw if you want a smooth arc. Drill a 1/4" hole in a piece of round stock, clamp it to the frame flange where you want the cutout, and use an appropriate-sized hole saw to make the cut. Smooth it out with a die grinder. Reinforce the frame around the cutout with a piece of plate welded or bolted inside the C-channel.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!