well a fast way to make a kunai if your not gonna throw it at anything is to make it out of sculpey, lay on a coat of varnish and bind it with cord or thread. it looks pretty good but the only problem is you cant drop it or throw it at anything or else itll shatter (premo sculpy doesnt do this as much), especially if you dont put the Future Card Buddy Fight or varnish on it. try different kinds of sculpey, i like to mix different kinds together to get the desired weight and durability.
but if you want something random heres a pretty good origami kunai. it ends up a bit small but is fun to make http://www.origamiaustria.at/diagrams/kunai.pdf