I'd bag the single tweeter idea. The whole idea of arrays for the DIYer is that you can get killer sound with very inexpensive drivers that individually aren't worth much but in large groups work very well indeed. I built a pair of these arrays for $60 total, and while the closeout drivers I used may not be available now there's always going to be something out there on the cheap that will work just as well. Using high end drivers, which with a single tweeter you're forced into pretty much, instantly robs you of the $ advantage.

I can't give details on my array prior to its publication late this year but I will say that for a $500 budget you should be able to come up with a lot more than you're currently aiming for by using bargain basement drivers.