use as much space as possible, meaning glass to every contour on the side of the trunk. However, you'll also want to use as much MDF as possible, since it is going to be stronger than the glass.
Once you have your back, then stretch fleece, or speaker grill cloth (I prefer fleece, since it gives you a stronger base) then back fill with layer upon layer of glass matte. Then make a mixture of kitty hair (bondo, laminating resin, hardener and chopped matte) and flow the inside of the box with it. This will assure a completely covered interior giving you a better seal, and thicker walls. You will need to strengthen from the inside rather than the outside, since you don't want to change the shape of the outside of the enclosure.
At this point you will measure internal displacement.
It's not about how much power you have, it's about how well you use it.