EDITED:
legs? On your immersion chiller? why? If I were you I would just bend it until it has the right amount of tubing where you need it first. You could build legs with 12gauge copper wire without the need to flux anything.
IF you need to you can use the solder and water soluble flux that you usually use for copper unions - just clean the **** out of it - soap, hot water, rub it down and do it again and again until you can't smell anything on it at all. Then I would acetone it and clean it again. But hey - that is me - all this stuff goes in your body if you don't get it first...