hot spots LOL!
Yeah sure, its *possible* The crinkle in the foil creating a lens that has the focal point perfecting on the leaf. Yup possible. (not to mention most alum foil guys are using CLFs, not enough intensity by the time the light gets back to the plant.)
No wrinkles, no 1 in a million chance of hot spots.
Mylar is *clear* It made shinny/reflective by guess what? *aluminum*
Just go flat bright white paint. Easy, cheap, and works just peachy.