If you want low, go with a larger ported enclosure (1.75ft3) tuned to around 28hz. Compare it in WinISD. Port on same baffle as speaker.

If the box is going to be a loose rectangle (not built to conform to the car), then just experiment with up or back. I went with up in my last hatchback because I used the spare tire well, but I recall getting more bass facing them backwards.