I'm getting ready to do my first topsy-turvy cake. Is there a specific type of cake that works better for all of the carving/slicing that's involved? I was thinking maybe a denser cake would do the trick. Any ideas? Thanks!

WASC, I would think. I use it for everything.

I would definetly use a dense cake. Box mixes tend to be way to light and crumbly, so something from scatch like you would use on a wedding cake so it supports the structure.
P.S. be careful if it's got things in it like choc chips it makes the carving a lot harder because they tend to catch on your knife and rip out little bits of cake.