I wouldn't go with the external blanket idea. Too much hassle even in good weather. drimportracing over at CMPG did a nice thread on insulating the hood on his Geo Metro.
Geo Metro Insulated Hood "how to" - CleanMPG Forums
Since you already have a fairly smooth underhood area, try the Al foil or Mylar idea. Al foil is cheap, but a mylar solar blanket is only a couple bucks and would provide one piece large enough for the entire hood as well as being slightly more durable. Mylar loses out to Aluminum by a couple percentage points in efficiency, but it's still over 90%. Since it's a radiant barrier and works by reflecting radiant energy, it's critical to keep it clean to maintain effectiveness.
I've been considering the mylar solar blanket approach as well. This idea could extend to any area where you're trying to reflect heat (undertray, AC lines, etc).