One of my best sounding setups was downfiring in a trunk. I had two 15's downfiring into the spare tire well. The box was raised about 2 inches from the trunk floor and the opening to the spare tire well allowed for good cabin gain, there was probably a 10in deep well they fired into. If you dont have a deep spare tire well then I wouldnt do it.
Down fire and port to the rear would sound great if done properly.
2012 F250 Lariat, lifted on 22's and 35's
4 DSS Ethos 18's flat wall, 2 Ampere Audio 7500.1's
Mechman 370, 9 batteries, Rockford Punch Pro front
Team DSS / Team Ampere Audio