Dang, that is the second amazingly clever idea that I've gotten off of this forum today. You guys are amazing!
I'll make one suggestion though… Rather than taping the edges, sand them, and apply epoxy to seal them. That way, you're actually getting more air in there, which will help keep the pads cooler. Just a suggestion.
@Demonizer: It's been a few years now so you should know if not having the outside edge of plastic weakens the revised backing plate at high rpm's/pressure. By the way, how did you find a machinist on craigslist? What were your search terms? (hope he sees this)