I have a clifford alarm with a proximity sensor and i have seat locks. If someone touches my soft top my alarm goes off. The seat locks are like wheel locks where it has a special key to remove them. You should also get a sticker to warn thefts that you have seat locks. Seat locks cost about $60 and are made specially for the s2000 by Mcgard. You can only get them through kodokan_4 on s2ki.com.

http://www.s2ki.com/forums/index.php?showtopic=216796