I'm thinking gumpaste. I've never made a cake topper before. Is it possible to make a 3D one? Or should I stick with a cookie cutter and make a flat one. What do I attach to it so it will stick in the cake. How far in advance should I make it. They want a name on it. Should I use gumpaste letters to put on top of the megaphone? Can I use buttercream and write the name on it before the party? Do I use pure gumpaste or 50/50 with fondant? Thank you!!!!

I would use either 50/50 or fondant with tylose. You can make as far in advance as wanted. You can use buttercream to stick it to the top or toothpick it on. As for writing it depends on what you are comfortable with. I hate my writing so I use fondant. Good luck

I made this one out of 50/50.
http://cakecentral.com/gallery/1408294/hsm-theme
I used a paper cone to form it on, the handle is on the other side. It dried really well and could go either standing up or on its side without losing shape.
If I was going to have it on its side I probably would put skewer holes in it before it dries (on one side) then after it is dried slide the skewers inside.
I don't see why you couldn't use buttercream to write the name.

3d sounds great! I agree, you can set it down with butter cream (or royal) or make a topper with a bottom to it. If you do the cookie cutter, make it thicker and run tooth picks (or we use bamboo skewers for a longer length) through it so it will stand up. I too prefer to cut our fondant letters because by the end of the day, I may not be writing so well! You can also use wires, but just remember that it not all wires are food safe and you would have to take precautions for that. (straws filled with chocolate works) Fondant mixed with tylose works well and usually does not upset the coloring. Make sure that you do it far enough in advance to dry in any case. I use a dehydrator to dry small items, but I read on CC where they use the oven with the light on. I have tried that on big items that do not fit in my dehydrator and it works great! I actually used it this week to do a stand up gymnast that is going out tomorrow! Good luck! HTH

I made something on a much smaller scale, but it was a topper with the school's initials. The megaphone I made was fondant/gumpaste mix. If you wanted a much larger one, you could make a cereal treat base then cover it with fondant. Fondant is heavy and likes to settle if you make a large piece with all fondant you may see your piece start to "buldge" or "squat."

http://cakecentral.com/gallery/2329209/cheerleader-and-graduation-cake

..one other thing to take into consideration in writing on a megaphone. The writing graduates in size. It is not all the same size. Larger opening side has larger letters.