I would say utility blade with a metal ruler to give you straight edges. may take a few runs depending on the kydex and cuts. for detail work I would thing a coping saw or jigsaw might work. sand paper and hot air gun for even more detail. a dremel is really cool to have around as well..

I did a search on kydex tutorials a while back and found quite a few dyi guides and threads. it's something I want to get into but just haven't made the time.