There's an extra large white one that comes with every apple product, I hear a permanent marker would work great.
Jokes aside, idk......google? eBay? I remember once I bought a band logo (The Offspring) sticker off of a website called stickerchick.com I'm sure an apple modification website will have something.
If not, refer back to the permanent marker and mark up the existing cover, if light still shines through, do it twice.
Edit: if nothing works get a regular piece of paper and trace the logo, it shouldn't be hard since it is not an even surface to the actual body. Then get a plain black sticker, attach the sticker and the paper somehow and cut around it. Take your time for precision cutting