I finally settled on a Kenwood powered subwoofer unit, model KSCW62RC. I fabricated a simple mount for it that sets it tight against the inside of the left rear fender in the trunk, right in the left side 'pog hole'. It has a wired remote, and produces a nice tight thumpy bass. This technology doesn't require cutting any sound ports at all. It was the lack of bass that drove me to something other than shelf speakers/subamp, and I didn't want to cut up the package shelf. I plan to add a small amp to augment the output of the head unit, which is a Blaupunkt Denver CD player.