Why does it have to be a special speaker for center channel? They make cruddy ones that fit behind the rear view mirror and even in a din slot, but do you want so much of your 5.1 coming from overpriced laptop speakers?
The "perfect design" is to have all 5 speakers identical with idential powering. This can be challenging dependent on the vehicle. Most of us have to mix and match.
In my case, I have a pair of MBQuart 6.5" coaxs from back in the day. One of the pair is blown. Looks like i have a great center channel speaker! The trick is fabricating the right enclosure.