The Shure Beta91A is optimized for kick drums and other low frequency applications, the Beta 91A uses a low-profile boundary microphone with an integrated XLR preamplifier tailored for a strong low-frequency response.

Are you using this piano for music?

Blow my mind, I've never used one of these but I can see where they are thought to work from some sort of POV, although I can't imagine it sounds good. Anyone done this before?
Shure Beta 91a http://www.drumics… Shure Beta 91a

Boundaries don't generally work well at all on pianos but it is done sometimes on rock n roll piano. The Helpinstill of course is the standard for live stage pianos. I don't recommend screwing anything to a piano that doesn't belong there. What kind of piano and what musical purpose are we talking about? Jazz? Billy Joel? Dr. John? Pinetop Perkins (RIP!!!!)? Vertical or grand? Bar band? Studio? Classical stage?

Several rules of thumb I use when I've got no time to mess around:
For a grand piano, 1 mic, situate it halfway along the strut with the lid full open
For a grand piano, 2 mics, situate one as above and one in the curve of the piano body pointed towards the middle of the lid, lid full open
For an upright piano, 1 mic, situate it between the legs of the performer pointed towards the middle of the lower kick panel (avoid the feet, but somewhere under the stool).
For an upright piano, 2 mics, situate one as above, and a second pointed towards the C two octaves above middle C from behind the piano (non performer side).
Lift 2kHz and 12 Khz by 1-3dB in the mix to make the piano "sing" a little better.

Add some chorus (or on a DDL - 50ms delay plus modulation and a little feedback) to turn a tuned piano into a honkytonk.

Hope this helps.

Velcro?