First of all, I would be very concerned about putting anything on top of a heated blanket, especially something like straw. I'd be worried about there being a fire danger. Also, there is a concern about too much heat generated by having both the Mylar blanket and the heating pad. The Mylar blanket is to reflect heat back, and with the heating pad, there may be excessive heat. This would be something you would absolutely need to monitor closely. Remember, a cat's temperature is well above ours, ranging from 99.5 to 102.5 degrees.
I would recommend you look at the answers to other questions on this Hubpage, as well as some of the comments & answers. Also, you might look at other alternatives to what's on my Hubpage, like Alley Cat Allies, FeralCatFocus.com or HomelessCat.org.