Watching with great interest! If the sheet metal corners don't work out, what about starting with a preformed starting piece, like a bbq, salad bowl, pressed pan or lid that you wouldn't have to shrink or stretch so dramatically?
Amazon.com: Concept Housewares 14 Inch Aluminum Wok Lid: Kitchen & Dining
Or perhaps there's a cast corner from commercial truck bodies that could fit? (Yeah, those are based on 90 degree corners,)
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Dunno if your metalworking skills have ever branched into lost foam aluminum casting? (Making em light enough might be a challenge, even in aluminum.)
http://www.youtube.com/watch?feature...&v=EyOoFDsrGr4
Good Luck!
-Jay