I wouldn't really worry about the diaphragm material. If you want something that can fit on ID horn bodies, which are made specifically for a car, then you'll want a 1" exit. I don't recall if ID has different mounting styles for the minis or full bodied horns, but I know the minis are 2 bolt, the fulls might be 3 bolt. Either way, there are horn drivers that use both a 2 and 3 bolt pattern.
Then, just find one that will play the frequency you want. My Celestion drivers are like tweeters on steroids. They'll play up to 20khz, but they'll also play down to 1khz with a steep crossover, which is how I plan to use them.