Tardis cake?

I want to bake a Tardis cake similar to this one:
http://www.doitmyself.org/2009/08/tardis-cake.html
for my boyfriend’s birthday (only half the size and without the interior though, sadly I’m not skilled enough to create such a masterpiece!).
I was just jotting down the ingredients needed and noticed that Leigh Henderson uses buttercream between the layers and to cover the cake before applying fondant and gum paste- but my boyfriend doesn’t like buttercream.
Can you think of an alternative that would work just as well?
I will try to present some buttercream in different flavours to him as I think he’s only had vanilla buttercream, but if he still doesn’t like it, what do I do?!