I made a rainbow inside wizard hat shaped cake in December.

It was more time consuming then difficult. I needed a rectangle shape so I used a lamington pan. My only recommendation would be to make lots of your mix up. I found that a couple of my layers turned out thinner then the others cause I hadn't made up enough mix!

Sandwiching them together was easier then expected. I don't see why you couldn't use the premade icing. I used buttercream and didn't put a lot in between each layer. I also used a combination of normal food dyes and gels. It looked awesome when it was cut.