I see lots of cakes with number toppers, and I have a client who want a large gumpaste 5 on the top of her daugher's cake. What's the best (and cleanest looking) way to attach this? I know some people use cookie sticks with melted chocolate. But I want the 5 to look nice all the way around, no ugly "back" to it.

Have you made the number already? If not make it so the sticks (I use bamboo skewers cut to size) are built into the number while it is wet .. that way they dry into the design giving you a way to attach them to the cake without having "attachmnet points" for the sticks on the back of the number.

As soon as I lay the number down to dry I put 2 toothpicks in the bottom. I have one in my photos of a number 2 that I did that with. Works great. Its the elmo one.