This box will get loud as hell and still sound pretty good. Make your box with the following INSIDE dimensions:
Width=30.5 inches Depth=22.5 inches Height=18.5 inches Your port opening will be 5.5" X 18.5" Your port length will be 16 inches
Use one end wall of the box for your outer port wall The inner port wall will be 16 inches long and 5.5 inches from the inside surface of the outer port wall (end wall of the box)
Mount your sub near the middle of the box equal distance between the inner port wall and the opposite end of the box. Your sub will face up and your port will face the rear. The rear face of the box should be approx 12 inches from the rear of the vehicle. Move your box back and forth until it hits the best. I think your setup will be louder if you mount your sub facing the rear but you shouldn't do that if you're inverting your sub.
Your box will be 6.25 cubic feet AFTER port and sub displacement
Your gross box volume will be 7.35 cubes BEFORE port and sub displacement
Your box will be tuned to approximately 40 Hz
Your sub will play well below box tuning so don't worry about the low notes
