How do you enslave Flak? Every time I try Rivet city turns hostile, I've tried following him through the night as he walks around the ship and sleeps but whenever I think the coast is clear and mezz him Rivet city still turns against me....any hints?

Don't get caught. Sounds stupid but that's it.
I've done it right in front of his room the moment nobody was watching. Wanted to do it inside his room, but there has always been someone else.

The one time i did it, i waited outside the market door at the shop's closing time. As he came out the door he appeared in front of me, and was an easy capture without anyone noticing.

It's best to catch him in his room late at night. Shut the door, slap the collar on him quickly and then run out of Rivet City. If anyone goes hostile, just keep running. As long as you wait three days away from the ship, people will be calm again when you go back.

i enslaved him in his room after shutting the door. hurried back to megaton and waited 3 days. fuuny thing was later on in the game i freed him from paradise falls. but now wont talk to me in rivet city. i guess being a slave does that.

Last time I mezzed him right outside his room and no one saw me do it. Just lucky that time I guess.

Last time I mezzed him it was in the stairwell and no one else was there. Easy wait 1 hour, then walk out like nothing happened.

Or you can do it in broad daylight, flee rivet city for 3 days or so, and everyone is fine.
Save before you do this for good measure

use stealth boys to get away. then wait three days before returning.

This is how I do it. Walk in, throw down, run out.