Here is a photo of a factory overflow for a '73 (I think) that I put into my '70 Barracuda. These will fit perfectly in your Challenger and were stock for the later E-Bodys.

YearOne makes repros that also look great. The bottle with the top and hoses will run you $60-80 and the bracket that bolts to the radiator and battery tray will cost you between $80-150. I got my bracket on Ebay.

I personally prefer this as it maintains the factory look even though they weren't available on your or my car originally.