How are you heating the ridgid tubing?
You could either use a hair dryer (which is kinda slow) or hold it over your stove burner (pretty high up so it doesn't burn). It doesn't require too much heat for it to get pliable. I just used an old filter sock as a form and bent the tubing around the rim of the filter sock so it is the right size. I used 3/16" tubing.