sorry but no way.
you could make a custom box, but why? you'd block off the spot where your rear strut tower brace would go! No way, that'd be full on rice. The harmonics inside the s13 hatch are great! just get decent speakers and a GOOD amp (clean power is important!), then add a sub... because of the shape of the cabin, my MTX Blue Thunder (with only a sealed 10" sub) sub and 300w amps w/infinity kappas sounds WAY better (louder too) than my friend's 900w rockford system, he hates that <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>
Especially since he paid over twice as much for his system, with dual 10" Infinity perfect subs <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/tounge.gif" border="0" valign="absmiddle" alt=':p'> Size is NOT everything in stereos... clarity and accurate reproduction of sound AND volume... now that's your goal.