Here's a simple way for you fellow IX Legion players out there to get assault cannons onto your attack bikes without having to chop them up. If you use a dremel to hollow out the back end of one half of a storm raven assault cannon (easily purchased on ebay), it slides right over a multi-melta. No glue or magnets required!
Now if only I could do something about the completely un-fluffy tyranid skull :/ Oh well, guess it's a demon skull now.