Just made my first mylar mirror today - not planning on making it parabolic, just wanted to reduce the weight and cost of mirrors. I'm planning on making the final one much larger, 4 feet by 6 feet. Note the difference in reflectiveness - I accidentally put it on backwards, so the back ended up being more reflective than the front. I used a cheap $2.00 emergency blanket for the mylar, which isn't that great quality wise; I might look around for mylar gift wrapping paper for the final mirror as it might be better quality. I also used heat gun to get out smaller wrinkles. Not bad for a first try!
05/01 - Update
Made a larger version using emergency mylar tent. 5'x4' frame using old 3/4" tomato steaks from last year and ripped into 3/4" strips. Super lightweight. Must remember next time to keep supporting internal members lower than exterior frame, and to sand exterior frame to reduce imperfections. Also note, this particular mylar is semi-transparent, which sucks to find out after building the whole thing. Still, not bad.