Do you have room to build an MDF baffle board to mount them to in the door?
Second, kick panels are nice for many reasons, mainly for minimizing path length difference (PLD). It will help with your staging, however there are downsides to kick panels too. Space, airspace, etc. Your midbass response will suffer in a kick panel unless you can give it enough airspace, or vent the kick panel to the outside world. Car speakers are specifically designed to work in a door, so when you go the kick panel route, you kind of have to mimic the airspace available in a door and build a strong non-flexing enclosure. I strongly recommend you do some research on that before you decide anything.
I can give you a nice link about the science behind kick panels, but it's very technical. If you are really serious about kick panels or a "proper front stage" I will provide the link.