Get a 4 or 5 inch dia. Helix coax car speaker (you may have to buy a pair, but so what? They only cost $120 per pair.) They sound far better than any of the usual small speakers you'll find in a typical upper-bracket Hi-Fi store. Put it in a little box with some fiberglas padding. I know that you've never heard of Helix - I just found the brand myself while looking for superb-sounding rear-channel speakers.
You can get them through "The Autophile" website. e-mail the distributor: he is very helpful.
You can get them through "The Autophile" website. e-mail the distributor: he is very helpful.