I would not use a cake mix straight from the box. It is too soft and "fluffy". You will want a denser cake for topsy turvy. You can try a WASC, which starts with a box mix, but you add different ingredients and it makes a denser, sturdier cake for carving and supporting itself.
I always add 1 cup of flour and 1 cup of granulated suger to my cake mixes, it keeps them moist, but firms them enough to stack and keep their shape. I have done 3D carving with this, but not anything huge.