Now you have many options once your speakers are wired up, or even before you have wired them up.
1. Buy custom boxes from car shop
2. Fiberglass a box for yourself (example below)
3. Take a pre-built box and use an indoor speaker mount to mount it to your car, it's Removable, twistable and easy! (I had these in first before I decided to fiberglass a custom box)
4. Get creative!
In the photos above, I'm about 2/4 of the way through making my own fiberglass speaker mounts. I still have to take that mold i made and fiberglass the speakers holes on it with some MDF circles and a sheet of fleece. Then sand the fiberglass and put some bonds and sand that. Then just paint and install!
Will update more about the custom fiberglassed box as I'm completing it.