I would have to agree with Skip that probably a few mods on a woolly bugger would be a fast and very easy way to get a somewhat crayfish looking fly. Another possiblity is to look at the Clouser Swimming Nymph (pattern is here and elsewhere), which with a few tying mods on the tail (maybe use rabbit instead of marabou) could give a nice imitation of a cray. There are only about 5 ingredients needed, and it is a very easy tie. On that application I would thing about a TMC 200R, or a Mustad C53S for a nice shape, but a streamer hook would work as well!
Good Fishing!