this is what i did for my front doors. I took the factory 6X9 and traced it onto some 3/4" MDF. cut a hole in the middle to accomidate your speaker. get some 2" screw with the same thread and your good.

If you look on the 1st page of this thread you will see that mtbcyclist has a picture of where he placed his front crossovers. thats where i put mine as well but the passenger side i placed high up over the floor area (where your feet would be if your sitting normally). you shouldnt have rear crossovers. if you run front components and rear components that would just be a lot of highs in your X IMO. juts gets some mids and your good there as well.