There is an option to get up to 3 of these in per fortification if you take a Void Relay Network, which can really allow you to dig in and fortify in ways we had only dreamed of. For each Void Shield Generator, you can add on two extras for an easy 25pts each.
The big question really comes down to, what do we make these out of. They are considered to be AV 13 Impassible Buildings with battlements. I suppose you could use a Vengeance Weapon Battery building without the weapon on it for the building, but you fall short a little on the battlements on top.
So the big question is, what are you thinking when it comes to modelling the Void Shield Generator?