Using square aluminum tubing is a great idea - it should give you a nice clean edge without too much fuss. You'll probably have to use some kind of filler for the joints around the corners though if you want a really nice finish.
I'm not sure if duct tape is the best way to go - I would imagine that it'll be hard to get the kind of tension you'd want in exactly the right place to remove the wrinkles when you're using tape that wide. The stuff I was using was roughly 3/4" wide and it was still hard at times to get out the smaller wrinkles. But there's not harm in trying, and if it works, would be a lot faster than using the tape I did - it too me roughly an hour to finish my large mirror. You're going to have to have patience when doing this, and expect a sore thumb the next day! As far as the hair dryer is concerned, it didn't do a thing to remove wrinkles from my mylar. I'm guessing that different types of mylar will react to heat differently, so you could give it a go anyways. The heat gun is just a nice final clean-up for the super-small wrinkles and creases, but if you don't have a gun, stretch it out first and take a look at it. You might find that it isn't necessary at all! Good luck and let us know how it goes!