I messed around with boundry mics back in the day. I had plexi-glass plates I made for the ones Radio Shack sold. One of the problems is that they have to be mounted on a large flat surface for the best bass response. The best recording I made with them was of birds and other critters outside my old apartment in Cascade, CO. I taped the mics separated by 10' to a very large window outside my apartment and just ran tape for a while. The resulting recording was very life like. I didn't like them for live music. I still have one of them that I did the balanced circuit mod to.
I just read this, and it so funny.
I did the exact same thing w/ my Radio shack pzm's.
They came with heavy metal plates.
I screwed them thin plexy glass, and added a mini xlr connector to it and made cables.
However, I did record a band by attaching them overhead on a drop ceiling.
It sounded pretty good. I'm not sure what happened to them.
I'll have to dig out the recording and refresh my memory.
I just remember having some success back then, and figure the technology had
progressed to the point of sounding great.