JZ,
Designing a crawfish fly isn’t difficult, but if you want to actually catch fish with it, consider this:
For crawfish patterns for bass, disregard how the fly ‘looks’ and concentrate on how it ‘moves’.
Bass are more attracted to the action of a bait than they are to it’s ‘looks’.
In other words, your crawfish flies need not resemble an actual crawfish.
Bass are instinctually attracted to two things that a crawfish does. First, alarmed crawfish will ‘flee’ by darting quickly backwards. Bass see this and are attracted by this flight response.
When the crawfish does this, the legs are tucked in and the claws stream out behind them, the tail is used for propulsion, but as they ‘glide’ they tuck it underneath. The tail has the heaviest muscle in the crawfishes body, so that part is denser than the rest, so that part sinks first. So, to ‘imitate’ this you need a fly that is weighted at the hook eye, has a relatively bulky silhouette, and has some legs/claws/antennae trailing behind all clumped together. (sort of a fat wooly bugger with dense tail and maybe some rubber hackle there as well).
To fish it, work the fly in short, fast strips. The fly will dart upwards then glide back down.
The other thing that many crawfish do that attract bass is ‘fall’ vertically through the water column. This happens when a crawfish that is feeding on/in underwater brush and vegetation becomes dislodged and ‘drops’ to the bottom. Bass are attracted to what they see as a helpless prey.
While ‘falling’ a crawfish will fully extend it’s legs, tails, antennae, and claws. They tend to fall with a horizontal atitude. To ‘imitate’ this, a fly should have long protruding legs, at right angles to the body, stiff antennae, and somewhat rigid claws. You want a fly that’s centrally weighted, so it stays horizontal as it falls. Use stiffer materials for this presentation, as you don’t want them to tail behind the fly, but hold their shape as the fly falls. Again, a bugger based fly with stiff saddle hackle for the legs, bucktail lashed together for the claws, and peccary for the antennae would work well.
To present this fly, cast it right next to submerged vegetation using a puddle or ‘pile’ cast so that it can sink vertically. Strikes come on the fall, so watch the line closely.
Taking the time to make it actually ‘look’ like a crawfish will be wasted unless it ASLO moves like one. Making a fly that ‘moves’ like a crawfish but doesn’t really look like one will still catch lots of fish.
Good Luck!
Buddy