Well, after seeing these replies, I decided to mount my mini-speakers into a 2.4 Ghz Cantenna. I take the Cantenna with me when I camp in case there is Wi-Fi in the area, so I thought it would be handy to also use it as a QRP speaker.
I drilled 4 equally spaced 1-inch holes in the back of the Cantenna and used silicon glue to attach the speakers. Then I used the Cantenna's cover to hide the speakers, using black electrical tape to hold the cover in place.
The new speaker seems to work pretty well, both on SSB and CW. I'm not exactly sure where it is resonant, but it does seem to peak CW signals at about 900 Hz. And I'm thinking that the Cantenna will continue to work on 2.4 Ghz as well.
I'm listening to a SSB QSO on 40 meters as I write this, with the speaker connected to my Elecraft KX1, and the audio level is good. And the background noise is much lower than what I am used to hearing on my base station transceiver, a pleasant surprise.http://www.flickr.com/photos/54524684@N08/6809880487/http://www.flickr.com/photos/54524684@N08/6809882411/
Thanks too all who replied.