This sounds like a good idea and abit of fun to make.
I would advise using some sort of smaller turret system that moves 360*
and either put it near the pilot or in the open top section that you are planning to have
tied looking about for some on
gw and forgeworld website cant anything fantastic
but you could put use the weapon turret that i have linked below and put it in the
open top section and add the twin-link weapon to it, it would look alright.
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat440244a&prodId=prod770020a
what would probably work better is if you just got twin link weapons attached them
together and then attach it to the tank in a way that allows it to swivel it 360 (or magnetise it to the tank)
maybe make look a bit nice and have an eldar model below it touching buttons on a screen to control the turret.