sunroof drain

Hi all. Can any one tell me why landrover made the disco 2 sunroofs with plastic drain boxes. Thing is, i have made tooling up to make these drain boxes out of metal and my intention, is to tack weld and weld in place in leui of the silly plastic axdition. This will hopfully prevent the reliance of adhesive to seal. Your thoughts anyone.

Cost and because they used the same sunroof as fitted to the D1.
Why change any thing, the drains are only to take away any water laying on the glass that runs of when the glass is tilted, so don't do that and u don't need them, but then who would open a sunroof with water on it anyway.