Port area (((aero vs slot)))

I finally have time to build a box for my two 12" type Rs. Ideally Id like to use aero ports, however I only have room for one 6" aero which really limits me on port area. My box dimensions can't be bigger then 20x36x13 and with a 6"x13" port I'll have 4cu @ 33Hz with 7" of port area. I read the general rule is 9-12 for aero 14-16 for slot.

I don't have the room to meet either requirements. If I build a slot port, in order not to sacrifice too much vol, my port area will be around 12".

So its either aero: 4cu @ 33 w/ 7"

Or slot: (24x3.5x11.5) 3.38cu @ 33 w/ 12"

Its a trunk application with only 1500rms. Id prefer aero for simplicity and weight. Need some ideas guys.