There's an old shade tree trick that I've used on occasion- - - -fill the tube with melted lead to keep it from collapsing while you make a sharp bend around a piece of pipe or something else with about the right radius, then melt the lead out after the bend is made. Very tightly packed sand can work, too, or maybe beadblasting media, but you need to really work a lot of it in there and pack it solidly.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!