I make those parts out of stainless steel, and use a stainless disc with the hole for the shaft a little off-center for a counterweight. With a little creative tinkrting with an angle grinder, you can shape the weight for the proper opening force on the flap as the spring heats up. The stainless flap, shaft, and counterweight will still be in good shape when your grandkids inherit the truck in a few decades.
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!