Depending on the scale and detail of your piece…
Make two halves in whatever manner is suitable. Formed sheet or
castings. Solder together like a clamshell. Its a good idea to have
at least a tiny drilled hole to vent water vapor that might have been
inside the halves. You would have great latitude in dimension and
shape. Use material thick enough not to warp too much when heated.
Bent tubing will work, the result will be uniform dimensions close
to what the tubing started as. There’s a limit to how small a radius
a tube will bend to without major deformation.
You might think of it in terms of inner vs outer dimensions. If you
want a truly donut shape(small hole, fat body), clamshelling is the
way to go. If its more like a bicycle tire, tubing will give more
uniformity and symmetry.
Good luck