Last week I made a 3-D groom's cake of two hippos kissing. Since hippos are relatively sturdy beasts, I figured that I didn't need to use any kind of special supports, and I'd just carve them using their blocky shapes and physics.
I started by placing the base cakes on the board and stacking them like a normal tiered cake. I had two 7" rounds as the base. I dowelled them and put the board for the next tier. Since I wasn't sure how much I was going to carve out, and I was going to insert another piece for the arms, I used a semicircle instead of a full round for the boards. I also didn't put the board all the way out to the edge of the cake in case I ended up carving some of the back part off.
The next step was to stack the 6" layer, then put in two plastic boards that would eventually support the arms. The layers were placed so that the front edge of the layers were all lined up flat toward the center of the two cakes. (The 6" layers weren't centered on the 7" layers below them.)