I'm making a cake that is two layer sheet. The top layer is cake; the bottom is a dummy. The top of the sheet cake will have jumping Dolphins. The sides of the cake will have underwater features. On the back, I want to add a "backdrop" that I can paint sky, clouds, birds etc.  Whats the best way to do it?  I thought a dried sheet of gumpaste, but I'm not sure. Would it be better to cover a cake board with fondant and let it dry?  I'm afraid when the gumpaste hits the cake portion on the back it will get soft. Have no clue how to attach either one. Suggestions?  Please......

I think that I would use a piece of cardboard box..so that the box flap could sit beneath the cake and extend up for your back drop. And yes, cover it with fondant.

Here is a tutorial on facebook:  

THANKS!!  I will check out the link, and I never thought of the cardboard leaving the flap in tact!