get an aftermarket CD head unit to fit where the DIN used to go? Are there aftermarket plastic pieces I need to buy to make it fit?

 

Either use a kit available from most car stereo stores or Crutchfeild (the Metra and Crutchfeild brands are the best if you go that way), or use the factory radio bracket and build a trim ring for your aftermarket radio (which looks the best if you hvae any mechanical ablilty) out of Acrylic or ABS plastic.

BTW use the aftermarket wiring plug adapter if the stock plugs are still there. It only cost $10 but ill make a night and day difference if you ever need to change the radio again.