I made a version of the setup pictured above using Alex Meade's tubing blocks for small oval tubing.



chainstay mitering setup

It wasn't the easiest thing to use but the results were good. If I had a drill press, I could have clamped this to the table and done it that way. Since I don't, I used a hand file.

I then joined the chainstays to the bottom bracket using a similarly rudimentary setup.




The real disadvantage of this kind of setup is that you have to check everything very carefully - there are 100 ways it can go out of alignment, out of plane, crooked, etc.