That reed valve is spring steel- - - - -a piece of flat stock, even stainless, is not likely to last more than a few minutes before it breaks from metal fatigue. I'd suggest starting with a piece of hacksaw blade, and grind it thinner in the area where it covers the ports. A solid carbide drill bit will probably be needed to drill a hole in it for a rivet or a sheet metal screw to attach it.
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!