My brother made a 8" former for a one & a bit inch tube, by laminating it out of plates -welding the plates together through holes in the sides (I turned the radius for him).
He fabricated another larger one using the inner half of bent sections of tube, by welding the quadrants to circular plates.
It is not too disimilar to what you are proposing, so yours should work if the tube has sufficient support
I'll post photos later
[edit]
In the attached picture you can see the holes in the face where a bead of weld holds the plates together.