With that space i wouldn´t go with a slot port, it takes way too much space, aero is definitely your best choice. Said that, 3" aero is WAY too small for that kick ass sub, i would try to use AT LEAST 2 of them , and as my boy strongman said, flares it´s a must be ! if not, you might get a lot of port noise.
If those were my measures, i would definitely get 2 4" aeros 17" long each one (full lenght original aero) , that would give me around 2 c/f and 38 hz tuning after displacement, if not, 2 3" aeros around 11-12 long each one depending on the kind of music you like, that would give you around 2.2 c/f and 35 hz tuning.
Since you only have 19" to fit the aeros, you might have to use them a little bit outside of the box (if you use the 2 4´s option) , if you go with 2 3" aeros then you don´t have any problems