non-circular nipple design advice needed

[PerKr]

For one of our products we currently employ a hexagonal tube for transporting drain-water. Short description:
At the top end, the drain-water passes through a nipple with circular inlet and hexagonal outlet into a plastic hexagonal tube. At the bottom end of the tube a nipple with hexagonal inlet and circular outlet leads the drain-water into a circular tube. The joint between the hexagonal end of the nipple and the hexagonal tube is not good enough to achieve a good seal and so I am now looking for solutions to this problem.
I guess I should either design a non-circular outlet nipple which could use our old tubing (the old circular nipples didn't fit in the limited space available, which is why the hexagonal design was rushed into production, but the tube is somewhat deformable and could be made to work) or a new circular outlet nipple and tube small enough to be used. Are there any basic guidelines or rules of thumb when it comes to designing circular and/or non-circular nipples and tubing?

thanks in advance