If someone were to ask me to construct a cornucopia for the top of a wedding cake (and this is what I actually do for a living) I would probably opt for a medium known as "modeling chocolate". You can purchase this stuff pre-mixed from alot of bakery supply companies, or, you can also make it at home yourself. Here's a recipe I found, but there are many others out there on the internet:
http://www.chocolate-recipes.net/site/chocolate_clay.htm

I would construct the item on a removable form, and then allow it to cool down (after the warmth of your hands working with it.) This modeling chocolate is incredible stuff...holds it shape, and is very user friendly. My partner and I just recently started working with this medium, and we absolutely LOVE IT!! It's much easier (and tastes better) than sculpting or modeling with fondant or gumpast.

Good Luck!

8) Hi. I'm new here. Tina Lofthus is right, a cornucopia made of chocolate would a lot better but just incase:

Yields: 12 oz

sugar-10 oz, Water-5 oz, Glucose-2 oz

-Boil in a pot till you get a light golden color
-Once you achieve that stage put base of the cooking pot in a bowl of cold water (no ice) and cool for 2-3 min.
-then lightly oil a rolling pin
-Dip a cut-off-wire whip (Like a whisk but no round shape at the end) into syrup and flick going around the rolling pin.
- once you get the amount you want start shaping it.

Oh! If you can't find any glucose I sometimes personally use corn syrup cause it's mostly used for fluidity (the flow of the sugar syrup)

Hope that helped.