I heard that magnets were the way to go, then I read that nipples were the best....Sooo, I use a combination of nipples and magnets. I attached (2) 3/4" magnets from Home Depot to a fender washer. The magnets fit nicely into the nipple and hold the washer against the nipple. They work awesome and they do not get lost. I have (3) 3/4" nipples with washers/magnets and (1) 3/4" ball valve. The magnets stick to the barrel while not in the nipple or you can insert them a little askew to fine tune the air flow or even with the flat side of the washer against the nipple to vary the air flow.
I keep meaning to post my UDS build photos but since I finished it, most of my time has been using it.