For the E30's a custom ported box would be the best without sacraficing the structural integrity of the metal plate between the back seat and the trunk.
Have the box up against the bottom of the rear deck and have a port made in the deck. This will reflect the bass off the rear window into the cabin. And it also keeps the trunk space open! If you want rear speakers just use a small set of 5 1/4" for rear fill and run them off the deck (don't over power them 'cause it will over power the fronts and ruin the "imaging").