I use the drop tube off of my No.55 charge thrower. The insert that goes into the bottom of the charge thrower allows my RCBS funnel to sit perfectly on it. With a little extension, it goes out to 30".
You can make your own, or buy one... Track of the Wolf has options as well as BACO. Longer the better IMO for the reasons Lead Pot showed, which is freakin awesome by the way! Great example of what compression does to the powder column.