Plumbers goop is what we use for both the soundboard and the striking surface.
The sound board sets the pitch of the tone. If you want a high pitch, you move the soundboard closer to the striking surface. The soundboard performs a couple of functions. Other than effecting the pitch, it also vibrates and makes a difference in the break over of your yelp.
Copper is an ok striking surface. Most call makers say it's something really special because they are simply trying to sell their calls. For copper to be good, the call maker has to make several dozen calls from it and find the personality of the copper within his call design. Once you find that the copper can sound pretty good, but it wont be better than a well made slate, glass, crystal or aluminum call.
Wood choice - Start out with the cheapest wood you have. You're going to be learning so you will mess up your first few. No sense in wasting expensive wood. When you get a design that you like and you're ready to make some beauties, go with the cocobolo. Finish is with an oil as cocobolo is a very oily rosewood.
"Instruments of Mass Destruction"