This is for the newer "magic band 2" for the Disney parks, which use the removable center "icon". While you can buy keepers from Disney, we wanted something smaller for my daughter Annie's charm bracelet. The print is fairly easy and quick, once you get the setting right, and, if printed with accurate dimensions, seems to pop pretty securely onto the icon. In fact, be sure to line it up right, because it doesn't rotate or come off easily. Also, be careful not to bend or stretch it too much, as it can develop cracks that could weaken it.
Used on 4 icons for a 10 day trip to WDW, with no issues, including an extra one that I had cut in two with a dremel (wanted to see if I could safely drill a hole for a lanyard - FYI: no, you can't), where the keeper was the only thing holding the two halves together!

Printrbot

Printrbot Simple Metal

Yes

No

0.4mm nozzle, 0.2mm layers

100%

I used ABS, with 85% flow rate for dimensional accuracy. 0.1mm gap between raft and piece.

Steam for about 2 minutes in acetone mist (gently heated) to give a nice glossy finish. Paint to desired color with fingernail polish or spray laquer.