The taper of this used Pentel pen looked about right for a traffic cone
The hole at the tip was too big and the cone was a little short. Cut & paste this link for details on how I fixed that.....
Fast forward: Here are the cones before paint. The bases are sheet styrene & 1/2-round rod.The insides of the cones were hollowed out with a Dremel.
Here they are in paint. The nice part is you can lay them on their sides and have an accurate hollow cone.
Lengthening the cone takes a little time, but the results are worth it.