heres my two cents:
Just get a dremmel, cut the OEM speakers out of their housing, and epoxy the adapter ring that comes with your speakers onto the OEM housing. Its really pretty easy, and the OEM housing is better than any adapter you could buy. It fits perfectly, is very sturdy, even has a hood in the back to protect against water, and you can even use the factory plug! It is really not that hard to do, and it just can't get any better than this.
Edited:
I forgot to mention: there are no drop in replacements. I bought some adapters that were very cheap quality and didn't fit well. The OEM housings are better than any adapter you can buy and are worth more as an adapter than keeping the stock speaker on your shelf for some reason or trying to sell it or anything like that. And for those of you concerned about taking your speakers out when you sell the vehicle, just pull your speakers out, replace them with an inexpensive set of coaxials, and tell the buyer that you have aftermarket speakers that are still better than OEM, and take your expensive components or whatever with you to your next car.