Okay here's the basics on what varius shaped attachments on an omni microphone will do-
They basically work like boundary mounting does, but as a special case in that the boundary is tiny, may not be flat, and the shape of any edge comes into play. Like "normal" boundary mounting, the size of the boundary determines the range of the effect. The larger the boundary, the lower the frequency range effected. In this case the boundary is very small, and will only affect a range of say 2kHz and higher.
Size of the attachment determines the center frequency of the on-axis response peak and shelf filter boost. Shape of the attachment determines the how smooth or peaked that on-axis boost is, and the behavior of the response off-axis, due diffraction around the shape and off the edges of the shape back to the microphone element.
A round plate shaped attachment (such as the DPA disk will be when suspended in free-air) will produce a sharper response peak than a spherical attachment, and less-smooth off-axis responses.
A plate-shaped attachment is less common than a sphere, but has been used historically by some manufacturers. Sennheiser is one I know of, although I can't remember the mic model. TS member and gear builder Jon (Naiant) used to offer an omni which he shipped with a wooden plate attachment (the Naiant X-W). Below is what he says about it on his website
http://naiant.com/x-w-specification/"The X-W is an electret condenser microphone that uses a 6mm omnidirectional capsule with a 36mm pressure waveplate attached to the capsule mount. The waveplate, fabricated from instrument-grade ebony or bloodwood, provides an increase in on-axis high frequency response which enhances the directional response of the X-W. The X-W’s circuit incorporates equalization to restore the X-W’s on-axis response to nearly flat, with a 2dB presence peak centered at 6kHz.
The X-W thus has improved on-axis signal-to-noise ratio due to the acoustic gain of the waveplate. The 90 degree off-axis response of the X-W is approximately 7dB less than the on-axis response at 20kHz. The X-W exhibits practically no off-axis rolloff below 3kHz."