This is a modified version of Jewcookie's freetrack IRclip. The attachment clip ("C".stl) is made to fit the band on a Sennheiser HD555, but may fit other headphones as well. To make it as light as possible I've used a single CR123A 1200ma lithium battery. The fully assembled IRclip weighs only 37g, compared to two AA NiHMs which alone weigh 57g. It's modeled around readily available RadioShack parts (links below). The switch is held in place by friction, and the battery holder is held in place by both friction and PLA "rivets" which you must melt after installing the battery holder. The three main pieces (A-C) snap fit together, the fourth "D" piece is an optional ring to hold it together permanently without glue, you must print two of these.
Switch: Catalog #: 275-007 (http://www.radioshack.com/product/index.jsp?productId=3020764)
Battery Holder: Catalog #: 270-080 (http://www.radioshack.com/product/index.jsp?productId=22874796)
UPDATE [12/2/15]: I've added some spherical diffusers for standard (narrow angle) 5mm IR LEDs. When lit with an IR LEDs the spheres glow evenly in all directions, but will reduce the overall light output. Works much better than wide angle LEDs in dimly lit rooms.
Printed in PLA with 0.10mm layer height (High res) and rafts, supports are not needed.
Try your own experimentation with shells and infill, I printed mine with 2 shells and 15% infill. I have not tested it, but 1 shell with 100% infill may be best for this job.
Note: Battery holder is held with PLA "rivets", which must be melted and compressed over the battery holder's mounting holes (or removed and the battery holder glued in place). I used a glass rod which I heated, and it worked flawlessly.
If you intend to use the optional locking rings ("D".stl), you must print two of them.
UPDATE [12/2/15]: Print three copies of the diffuser in white or translucent PLA. They slip-fit over 5mm LEDs.