kevmurray

A horn can't be beat for efficiency. Nothing comes close. That said, the box you mentioned is too small to take advantage of a pair of 15's. The horn length is too short and the mouth too small to get low. A proper bass horn would take up the whole bed of the truck which isn't a bad idea if you want to compete. It would be fun too.
Other choices are a bandpass vented into the cab, or a simple ported box. The bandpass would only require a few holes in the cab for the ports to pass through. The ported box requires a large rectangular hole but it sounds better.

If I had a pickup I'd build a huge horn and fold it vertically. Make a giant scorpion tale or something. Okay I'll stop now.