If you already have a specific amp you are trying to get in there this wont help but if not...
Amps that are behind the passenger seat include:
Lanzar Opti 400 (Truweb's post):
http://www.elisetalk.com/forums/showthread.php?t=13336
I have the Lanzar in my car - note that it has to be raised off the floor to go behind the seat as in Truweb's pics - there is a rectangular cut out in the back of the seat (at least in my 05 Elise) which my installer did not realize and had fits trying to figure out how the amp would go in there... You get more space up high rather than at the base as a result.
Audison SRX4
http://www.audison.com/default.asp?lang=eng
Gemini Profile 4
http://www.theautophile.com/products/ProfileFour.html
If one of these will do then you need not mess with the seat.
................... Gary