I looked at the ways that are suggested above when the weld broke on my rear window while I was replacing the glass (with Lexan). All it took was one high speed baseball to shatter the original glass!
I tried using an L bracket made of aluminum but couldn't get it to tighten up using aluminum rivets.
Then I remembered that the West System G/Flex epoxy has very good aluminum adhesion properties. I roughed up the channel with 80 grit sandpaper and put in several layers of fiberglass embedded into the G/Flex. Looks and feels like it will never come apart in a million years, and the corner is permanently waterproof too!