Short "low boost" grids, or no grid at all (you can re-purpose the long "high boost" grids into the acoustic equivalent of the short ones by popping the screen out and sanding down the tube until it is the same length as the short ones. No need to replace the screen. Using no grid at all provides the flattest response, useful up close or with no windscreens. I'd probably do that inside a piano. The short grid is what I use and more or less compensates effectively for high frequency attenuation of a windscreen or fabric. The long grid, intended for voice use when mounted under fabric on the torso is useless to me for recording music, as its boost response is too much with an overly high Q. The mics are typically provided with one short and one long grid.
I generally just use gaffer tape for mounting them, they are so tiny and light.
I've found the boundary mounts useful, and primarily use them for mounting under fabric without creating a lump and providing a smoother surface for the fabric to contact, reducing potential rustling noise. They are not really necessary for boundary mounting to a hard surface except that they provide protection and make it easier to mount the mic to the surface by providing a large surface to tape. I use them when laying the microphone on a stage surface as it will protect the mic from being crushed if someone were to step on it. Otherwise you can just gaffer tape them directly to the surface.
The DPA concealers are similarly useful for mounting under fabric. They are considerably smaller in diameter than the boundary mounts and reduce fabric noise a bit more but are not as flat. Happy to discuss some stealth techniques for classical music with them that work very well via PM if you like.
I use a pair of black telescopic TV antennas as mic bars for these, connected to a center hub. They are small, light, nearly invisible and telescope compactly for transport/storage yet a pair will extend to 5 or 6' wide for larger AB splits when appropriate. A small band of gaffer tape at the end of each extension section secures the cable and allows for extension and retraction as required. When retracted the cable forms a few loops between sections, when extended the cable is taught against the antenna. Can tape the smallest extension section or two closed to limit full extension to whatever is desired and minimize wiggle.
I've made DIY APE (acoustic pressure equalizer) spheres for them which I use in various ways in my open rig. They are act like the APE spheres available for 4006 and the like which are about the same diameter, and serve to convert the nearly truly omnidirectional polar nature of these tiny mics (depending on what they are mounted on) to being directional at high frequencies, with a bit of a presence range boost on axis and high frequency roll off off-axis. So truly omnidirectional almost all the way up on their own, or a Neumann-M50-like pattern when mounted in the spheres.