I pick up every Town Car aluminum temp wheel/spare I can. The tires don't have a lot of the UV protection road tires have, so they have to be kept out of the sun, but they make it easy to move around my 63 Falcon. I also keep one in my 2000 Grand Marquis, one at the house (so I can put it on when I have a flat at home). I had the weights of steel versus aluminum someplace, but can't find it. It is enough that it makes me think my wife could get it out of the trunk (I don't see her changing a flat though).
I have a 17 inch temp spare from a newer Mustang someplace, in case I ever install the Cobra brakes on my Mark VIII, and do a hub swap and swap out the wheels as well. The Mustang 17 inch one would fit your car as well.
Trying to get a 17 inch temp spare for the smaller mn12/fn10 bolt pattern has been a failure, Volvo 17 has too much backspacing and hits the caliper, Lincoln LS 17 is way to large for the space in the trunk (and is actually taller than the tires I actually run!).