More likely than not, you'll find that the spare tire well is too small to fit a decent sounding sub-box. That, plus the fact that you mentioned that you have a small 3rd row child bench sitting on top of it--even if you did manage to shoehorn a small sized sub-box in there, you'll probably not hear a whole lot coming from it. Subs need a reasonable amount of airspace to sound decent (the sub manufacturer will give you their recommended box dimensions and required airspace), most people who want decent performance from there subs, expect to lose some trunk or, in your case, hatch space.