Sousaphones are made just like any other brass instrument... only BIGGER!
Seriously... those large bows like you find on a sousa bell are usually made in two pieces and 'clam-shelled' together. The flare is then spun and attached as a separate piece.
Dan Schultz
"The Village Tinker" http://www.thevillagetinker.com" target="_blank
Current 'stable'... Rudolf Meinl 5/4, Marzan (by Willson) euph, King 2341, Alphorn, and other strange stuff.
I think the elbows for the back of the bell and the bell receiver are either cut and shaped that way or formed in two halves. You can see both methods in the link below. It's about a saxophone being made but the idea is the same. Sorry about the wacky music.