Merit is another company that makes a very good adjustable iris that attaches to your eyeglasses with a small suction cup.
Works very well with getting that sharp sight picture with irons, but is a little "fiddly" till you find the right position to place it.
Flyfish is right in saying that most serious bullseye shooters today are using red-dots and the Ultra-dots are a very good choice. The 4-dot versions give you the ability to not only choose the dot intensity, but also the size. They also come with a polarizing filter which is a great tool to help when shooting outdoors in very bright light.
ps: The Ultra-dots will also hold up well to the recoil on your Gold Cup.