If you want to amplify the sound a little more from the chopper, you can connect horns to the holes in the rotor. If you can, you can use a small plastic cone, trim the bottom off and the top and the glue it on. You can make the siren out of wood, but you would have to weatherproof the outside, probably by using sealer or covering the outsides with meatal, like small sheets of steel.
One thing you really need to make sure is to balance the chopper! If you make the chopper by wood, you can do this by drilling the center out of your chopper and shoving a large dowel into the hole. If it falls in one direction, use a small spade wood bit for a drill press and make a small hole on the side that falls, and repeat the process until it stays upright. (Make sure you do this on a level surface!)
I'm not sure if that's a surefire way to balance a chopper, but it should work!
here are some videos you can use to get an idea of how to make a siren out of wood (not my videos!):
https://www.youtube.com/watch?v=_CsV6LxhtTk
https://www.youtube.com/watch?v=ncBKC8gtT2k