needs a high-tech approach
I think the enrollment machines should be roughly robot-shaped, with at least a swiveling head and glowing red eyes. It should have one of those low-powered sweeping lasers they use with fog machines to spice up music videos. Give it some motion detection capabilities so they can track and sweep punters as they pass by and (obviously) a metallic-sounding voice circuit and a limited repertoire of phrases like "lifeform detected", "halt! please submit to scanning!", "anomaly detected", "uploading scan", "please remember to report suspicious individuals", "be pure, be vigilant, behave!" and the like.
The great thing about going about it like this is that the original machines could be no more than toys, just randomly going through their repertoires. Then, once the public has gotten used to the idiocy of the machines, TPTB could begin to gradually and covertly upgrade their capabilities so that they actually do the biometric scanning that the earlier models were only pretending to do. By that stage, anyone who had any clue about what was really going on and tried to warn anybody would be treated with derision, because obviously everyone would "know" that the machines were nothing more than electronic scarecrows only designed to scare away terrorists, paedos and illegal immigrants and instill a false, but reassuring, sense of security in the general populace.
In stage three, the machines could be upgraded to listen in to gossip at the local post office, supermarket, etc., to identify any troublemakers who may have cottoned on to the plan. Then the controllers could use the machines for more direct psyops, suggesting to neighbours that such-and-such a person seems to be behaving strangely this week and wondering if they have something to hide and so on. By then end of stage three, all the tin-foil hat brigade would either be driven insane by the whispering campaign or have fled the country.
Bringing us finally to stage four... a techno-utopian panopticon, where nobody has anything to fear because there is finally nothing left to hide.