I (thus far) do all my tube miters by hand, for two primary reasons:
I don’t have a better way.
It’s still pretty fast, once you know what you’re doing. I don’t yet…getting there though.
So again, made a thing to make other things.
This fixture will at least allow me to get the rear triangle seat/chain stays ‘more’ symmetrical w.r.t. each other…which should mean better symmetry w.r.t. the bike’s ref planes.
AND IT’S NAME IS MITER MOTHER!!!!