First, I doubt that the mount has to be welded all the way around. They
seldom do in HSM projects.

*Assuming* that they do not, here is what I would do (it's what you
might call a vertical plug weld): on your mount, before you cut out the
hole for the tube, drill holes (3/8 diam +-) around the circumference of
the hole, centered on the circumference. Maybe 8 (every 22 1/2 deg).
When you then cut the hole for the tube, it will have semicircular
notches around it. Fit the mount to the tube and fill in the notches
with weld.

Let us know if you try this and how it works,
Bob