For the restriction on dimensions, as far as I'm aware there is no other sub that offers the same level of output in such a compact size as the JL Audio E112. If
@bacapicker can spare another inch or two depth and height, that would open up many more options for better values -- the Rythmik LV12R, the Outlaw Ultra X12, the RBH I-12/e, or the XTZ Sound SUB 1X12, for a few examples.