I usually use a 50/50 mix of gumpaste and fondant for sculptures. It takes longer to dry, but is easier to work with and has a stronger consistency once it is dry.
if the sculpture is for saturday, when would you start this so that it has enough time to dry?
I would shape rice krispy treats for the inside and then mold the 50/50 gumpaste mix over it to shape the head and ears. You could then use wire stuck in the head to shape the antlers and mild the gumpaste over the wire. Modeling chocolate (candy clay) would work well as a medium too, but definitely use RKT's on the inside because all fondant/gumpaste would be a lot heavier.
I would start early so that if there are any problems you have enough time to work throught them if you have not done this type of work before. I learned that the hard way...
I did an elk like this, and I used modeling chocolate. SO EASY.