I am not sure if this is has been on here before, so I will post it.
I found a spare tire from a early 90's Jeep at a local junkyard. It fit on a 77 Aspen that was there, so I figured it would fit on the Challenger. I have not had time to test fit it, but I just wanted to share the information. The spare is a 16X4 with a 125/90/16 tire. It is about 24 1/4 inches tall and should be a good option to have more space in your trunk. The best thing was that it only cost me $13.43 for the wheel and tire.
Here are two pics: