Yeah. I was thinking I might just shim the dog ears with some wood shims on the back to get it snug fit. We shall see. Would be nice if they could be righted from both sides.
A shim might make it tight once, but the vibrations will make it move. Look for speakers that have a clip which snaps in place as the speaker is pushed in. I know Dynaudio has it, but haven't looked for others. BTW- Dynaudio are great speakers, but they're expensive.
Assuming you don't want to go up to the ceiling because you don't like being that high, which I can understand, a small man-lift is the safest way to go up to install the speakers. However, 20' is awfully high for speakers- the reflections could make the sound unmanageable.