I've had no problems with a standard cake mix however I add an extra egg to help add to the density. Just be sure to follow the directions on cooling afterwards.
I think a good choice would be to use a pound cake mix(betty croker has one) along with any regular cake mix. That should give you enough batter.
Also...there is a good pound cake recipe on the side of Duncan Hines yellow cake mix. I have used in in the 3D duck and it worked well.
I used this recipe
http://www.cakecentral.com/cake_recipe-1599-53-Enhanced-Cake-Formula.html
when I wanted to test a firmer cake for sculpting but not have to use pound cake every time. My only addition next time to this recipe will be some vanilla or other extract. I thought the resulting cake was floury tasting. It did produce a nice firm and moist cake though.
Here is another one that is in the recipe section here:
http://cakecentral.com/cake_recipe-1972-23-Durable-Cake-for-3D-and-Wedding-Cakes.html
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