I used a Columbus tapered tube. I only cut the narrower end and left the bottom as it was. I took it to a local place where they make motorbikes hoping he could face the top on a lathe (I didn't have my minilathe at the time). He couldn't, saying he would have needed to make a plug to go inside the tube to hold it properly at the chuck (which sounds completely believable), but he managed to do a very good job with a belt sander, a machine table and a square.

I then had to modify my coping template calculator software to be able to handle joining the DT right onto the tapered part (you input the ratio of diameter change to length change across the taper). This part actually worked really well especially as I was fresh out of tapered holesaws.

The way the Columbus tubes work is you then braze some inserts into the top and bottom that support the actual bearings. This possibly makes facing a bit less critical. I chose to braze (silver solder with MAPP, not pretty but they aren't going anywhere) those in after welding the frame concerned that the welding might melt the braze. But I think this was the wrong way around. The HT distorted a bit during welding to the point where it was compressing the inserts and the bearings didn't want to go in. This needed some last minute squishing in a vice to correct. Brazing them in first would have added support to the tube and reduced distortion in the first place and probably nothing would have melted. Those HTs are pretty much on the 50:1 diameter:wall limit considered the rule of thumb for coke-canning.