I used one of the chartreuse green wiffle ball bats from walmart (the really short fat one) the past three seasons. Drill out the end with a 3" hole saw to leave a little lip around the end. This gives enough back-pressure to get a good guttural sound out of a diaphragm. Cut the small end off just past the enlarged hand stop. Chamfer the inside end somehow (I used a large counter sink, but most may not have one big enough) and use a lighter to smooth all the cuts up. Drill a hole near each end and use some shock cord for a lanyard.
That being said, it doesn't paint up well and the chartreuse is pretty offensive in the woods. Instead of paying $15 for camofoam tape, I just bought a Jacobson RockyMtn one for this season. We will see which I prefer. I think the tone is better on the RockyMtn one, but the home made one is definitely louder.