Honestly, probably rather badly. But never let reality get in the way of a good story.

There is a brief description of them towards the tail end of the story, if you missed it. The iris I picture as something similar to the one from Stargate, though smaller of course, and explosively rather than mechanically shut. Excess explosive power launches the head. There's probably no way to fit one in a collar of the given description, to which I say whats your point, it's bloody fiction. Emphasis on the bloody bit, hehe.

For the selection process, well it's amazing what you can do with computers, radio communication, small GPS units, and a randomizing algorithm.