Hi Tiger,
I am in the process of converting one! One of the resources I found online was this.
http://www.bolterandchainsword.com/topic/192870-scratch-built-thunderfire-cannon/ I'm not a fan of using the lascannon based gun, but am mimicking the chassis to a certain degree.
For your situation, putting 4 similar guns together will probably work. You could also just use the quad cannon! This however is by far the most basic one I've seen built.
http://creativetwilight.com/homemade-thunderfire-cannon/
In my conversion, I'm using the Imperial Guard Ion Nova Cannon as the main gun. I've also ordered a Hellhammer Cannon piece from the Baneblade for a size comparison.
This, combined with the new Stalker/Hunter tank, I've a feeling there will be a lot of stand ins and conversions popping up!