A 300 cover will not work with your 505, however a 600 cover might. I've owned both readers, though not at the same time, and they appeared to be very close in size. Perhaps someone who owns both can enlighten us.

There was a light wedge version made for the 505, and I still have one. But it was total crap. It had a thick piece of translucent plastic which used LEDs to illuminate the screen and laid on top of the screen. It did illuminate the screen but the transluscent plastic made the screen look fuzzy and blurry and made it extremely difficult to read.