A duck pan is a magical device for making something change into something else, or disappear. A duck pan will fit a duck, a dove pan is smaller and will fit a dove. In the case of our Bubba the BullFrog, he fits the duck pan. So you can try to make a balloon animal frog from a little green balloon and after failing to blow it up, you can say oh well, there...a tadpole. That's close. You can then throw it into the bottom of the pan and talk about what a tadpole needs to change into a frog, add "water", add "food", a little time and Ribbit, you change the tadpole into the bullfrog puppet (secret not revealed here). You can then remove the big frog puppet from the pan and go into a little routine with it, or just take a bow. It's totally what you make of it. There is a photo of this pan at
http://www.axtell.com/illusions.html
A voice for a frog could be anything. Kermit sounds nothing like a real frog and it works fine, so probably any character voice will work that is different than your own. If you want it to sound like a real frog, start with the basic frog sound. Ribbit, brwawk, knee-deep, etc. Then form words. Here is a video clip of me doing our bigger frog with a voice idea...
http://www.axtell.com/video/frog.mp4