My thought was that the Lurker would be less like a big tank or armored assault troop and more like a strike troop designed for mobility. Big weapons like Plasma Cannons don't really suit what I was looking for, but Plasma Rifles are perfect: light, powerful, good for being a nuisance, and already shown to work with mobility (Ghosts, Banshees, Elites, etc.)

As for the vulnerable spot, you could have a thin open space running down the middle or 'spine', which would be hard to shoot unless you had something that could invariably hit it, like a Shotgun. Unfortunately, I think it'd be pretty hard for me to add a thing like that at this stage, because I had to merge all the layers to get the thing centered (this was done in Sketchbook Pro), and failed to save a 'workable' copy. My bad.

Vale,
Perseus