I have the Peerless adapter, but haven't installed it yet. I've ordered a Definitive SSA-50, which I intend to install under the monitor.
It has a kind of open "U" channel, with the U facing outward. This will accommodate a convex back on the speaker. There are two sliding plates through which you can insert a (M8, I think?) bolt to mount the speaker. Sorry, I'm doing this from memory, as the adapter is downstairs in my storage. (I need to dig it out, since I just ordered the speaker...)
So, if your speaker has two M8 (check the drawing on the Peerless website for sure) sockets that are centered vertically, you are good, because the sliding plates will accommodate any width between the bolts.
One disappointment is that the adapter didn't come with the right hardware to mount it to my mount. I have the ultra-thin one, SUA750PU with a thin Samsung plasma. You can use the adapter if you mount the vertical strips to the monitor using the supplied (with the mount) spacers. You'll have to go to the hardware store to get some nuts and bolts to mount the fixed plate to the mount. There are large holes with adjoining slots that line up that you can use, but you must use the spacers.
The piece with the U-channel then bolts to the fixed piece, which has threaded holes. There are a number of holes that allow you to adjust the vertical position to align the speaker to your monitor with no gap.