You can buy spring steel and make your own. I found it difficult to cut without putting slight, but permanent bends in it though. I ended up sucessfully cutting one with my CNC machine. A cutting disc on a rotary tool would probably give much better results than scissors. You would then need to paint it and rivet it to the bracket.

I don't remember the thickness I ordered. It might have been .007". You'll end up with a good amount to experiment with.
http://www.amazon.com/dp/B00065V03K/ref=biss_dp_t_asn

I just happened to come across this site where you can order custom ball flaps. You might prefer going this route if you don't want to mess with cutting on your own. It looks like you would still need to rivet, though.

I have no affiliation with the site, nor do I have any idea what kind of quality they are.

http://www.scvgames.com/items-for-sale/details.cfm/pitch-and-bat-ball-flap---new/id-90