Shoulder Massage Holder-kun

Shoulder Massage Holder-kun (肩揉みホルダー君) is a contraption sold in Japan apparently used to massage the shoulders of its wearer.

Principles
It is famous enough to be advertised in the top-selling weekly manga magazine in Japan and in late-night mail order television. The device looks like a U-shaped plastic device, which was probably around 15 to 20 centimeters long. The U-shaped portion would fit directly over the shoulders when it was used, and came in an economic set of two (in the magazine advertisement).

Although Touma deemed that Fukiyose Seiri might be into the contraption as well, it is only shown in Toaru Majutsu no Index II.