<font color="blue">"Hey that sounds like the ole soda can,tennis ball bazooka. anyone made one lately? " </font>
You gotta tell me about this one, tennis balls and lighter fluid? How does that work? We used to use firecrackers (thats before fun was made illegal) under a tin can and launch them into the next county, but tennis balls is a new one to me.
As for a good flapper, of all the things we used what worked best was a cast iron 2" (or smaller) threaded fuel cap, the kind used on fuel storage tanks. It would fly open on start up and stay put with no rattles. You had to manually flip it closed though because it was not spring loaded. My dad threaded the exhaust stack, but for the thinner stacks used these days a threaded coupling welded on would work.