I don't understand what people are getting het up about with the skull cannon. The only thing that should be on your mind is that it looks stupid.
It's only 35 points less than an ogre ironblaster and that gets S10 shots as well as S10 grapeshot. Yes the attacks are flammable and yes they're magical, but lets face it, it's the only decent khorne option in the book, as experiment says, Crushers for their points are pretty abysmal, and in the contesxt of the OP's army will be pretty much on its own especially with the comedy chaos roll that you have to do every round.
I think the book has bigger issues than one cannon causing hysteria - BoTWD hysteria anyone? Yes it may be cheap, but in the context of an army book which encourages through its mechanisms, but in the same breath makes it unfeasible (if you're a competition hound) to run a mono god list (unless its nurgle :p), surely they can be forgiven for having one shining star?
Or you can do what everyone else does, not that I like it personally, but get a nurgle model of choice, give it a gun barrel and call it a nurgle themed skullcannon.