Shim Stock works pretty well.
I've seen them made out of Carbon Fiber.

You can bend the stop. This does allow the reed to open wider.
Just remember to change them.

If you fly a lot, what ever that means, change them every year.
If not, change them less often.

It really is a crap shoot.
Unless you need that last 500rpm I'd leave it stock.

I tend to put powerful engines on my planes and fly at 1/2 throttle.
Instead of using a small engine and flying it at full throttle.

Here is another cage:
http://cgi.ebay.com/REED-VALVE-RETAI...ayphotohosting

He may have the reeds for it also.