LOWES sells blue electrical boxes made out of some kind of tough plastic. They make great horns and the 90 degree angle is already there. I've been making home grown horns out of gutterpipe for years on 40 sized airplanes, but for the larger planes, guys are going with the electrical box material. Amazing what you can find by walking through LOWES in the right frame of mind
..guys are also making horns for the larger planes out of 4x4 PVC fence posts.