My personal favorite option is something I saw in a very customized install, done in a 124 car. The guy got a box with ports on the top, and just added a port extension that went through a 2" hole he put in the first aid kit space. Otherwise, I would take out the rear speakers/cut holes in the first aid kit space, and get a bandpass box - the reason I would choose this box is that the majority of the sound comes through the ports themselves, so if the port was under a hole you would get a lot of sound in the cabin though. Bandpass boxes are touchy though, make sure you get one that is designed for your sub (contact the sub manufacturer). OTherwise just line up a ported box's port under the hole.
If you get a loud enough sub, you will hear it well with just a hole in the first aid kit - i have two audiobahn aw1200's getting about 500watts rms each, and it absolutely slams, and even 1 12" getting 300 (on my old amp) sounded fine under the first aid kit hole. Good luck with your setup - remember that the bigger a hole you have in your rear deck, the more road noise you will get.

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81 280e
16" 8-holes
clear indicators, new exhaust/intake

stereo -
jvc head unit
6 infinity kappa coax's
aura mr675h 90x6
2 aura mr475's, 200x2 each
2 audiobahn aw1200s in a ported box
1f cap

not bad for a car older than me, huh