I've used ones like you are talking about made from birch bark they work good for both bull & cow calls Another trick is ti carry a shoulder blade from a deer, I use one from a caribou. Use it for both visual and sound; you use it to imitate a bull thrashing his rack on trees or use it to flash some color he'll think its another bull out there. One more thing if you are around water pour water in water so it sounds like a moose peeing in a pond.

Watch and listen to them when ever you have the chance and learn all the sounds they make then learn to imitate them.

Good Huntin'