KobraCast is excellent for making lightweight horns from any form or shape.
Skymone Cosplay used KobraCast and expanding foam to make her Ysera costume, and shared this tutorial of the process with us.
You can see the same process in action from these photos by Lightning Cosplay
Lightning Cosplay also used KobraCast to make copies of actual antlers for her Faun costume, and recorded the process.
“I just want to show you this fantastic material, it is perfect for big horns like this because it is very light weight.”