Damn, $5 and works!? Awesome score.
A custom paint job should cover that up nicely. lol
....
Joking aside, if the blemish reacted or melted into the plastic, then goo gone probably won't do a whole lot. If it's just on the surface then you might be lucky to get it off.
I have a PCE Duo that someone splashed something all over it. The damage on mine was probably a lot worse then your Duo. Some parts cleaned off but other worse parts etched/melted into the plastic. I tried a lot of different cleaners too, even goo gone. My last resort was that I ended up using high grit wet sand paper to even the plastic out which blended the etched parts some and made it look a little better. Unfortunately the plastic did become smooth, losing the original matted texture/finish.
I eventually swapped the shell since I was very fortunate that member bluebmw hooked me up with a nice replacement. I would offer you my cd door off of my old shell, but yours looks like it's in a lot better condition. Good luck.