This cyclone missile launcher conversion is one of the simplest conversions I have done. All you need is a typhoon missile launcher from the Space Marine Land Speeder box and some green stuff. So as long as you don't require a Typhoon-pattern speeder, or you have a friend who doesn't want that type of speeder, you are on to a winner!
For the next stage we need to assemble the missile launcher and trim the missile feed to the launcher down to a suitable height. With my launcher I left a couple of the missiles visible in the feed in place. It is possible to take it right down, but all modelling is down to your own preferences and how you want your own models to look.
The last and final stage is to use the green stuff to create a lid to the missile feed to close it off and to hide the cut edge from trimming down the feed. Leave the green stuff to go solid and voila, a cyclone missile launcher. With mine I have used a magnet in the bottom of it and the top of the terminator so that I can have a bit of flexibility in list writing depending on what I want to do.
And that's the sum of it. There really is nothing more to it than that, apart from adding on insignias of your own army and painting it up. So go forth now, and let those missiles cleanse the enemies of Humanity.