I'm the kind of guy that when he has a building matter, walks around the hardware store looking for a solution to a problem. I ordered a Megatron cone from Horizon hobby. Here are the parts. 2 1/2 inch PVC coupling (Electrical grey PVC) a 3" Test plug (in plumbing) 3/8 x 3 1/2 inch carriage bolt. (this replaces the bolt in the test plug and makes it long enough for your drill) and one of these from Horizon. There is a raised area inside the PVC coupling that needs to be cut off. I just put it all together without the silicone adapter and ran it on my drill and cut out the raised area with an exacto knife. You could probably sand it out as well.

http://www.horizonhobby.com/products/SUL636