What you could try, Home Depot has a tubing bending set. these look like tightly wound springs that are flared out on one end. I found them in the tubing are in the plumbing section. I think they go from 1/4" tubing up. Anyway you slide the tube in the spring and then bend it and the spring will not allow the tube to distort. Then the hard part is getting the tube out, you just have to twist the spring off the tube.
I would try these you should be able to get the radius you want. Also try the soft copper tubing they have I think the biggest they come in is 3/8" but the stuff is soft and pliable, it comes in rolls. It's is the stuff that you would use for like the Ice maker on a fridge or to run water to the swamp cooler (if your from a part of the country where they have swamp coolers
-Mike