I was able to get my 505 cover (the standard Sony one) to work on my 600. It required a bit of patience and good bifocals, but I got it to work without much difficulty. The problem is that the 600 is slighlty longer (top to bottom) by maybe 1mm or less, so you have to work at getting to cover to click into place. Mine kept wanting to slip off to one side. Use your fingernail to pull the plastic tab that holds the cover in place outward a bit. If I can do it with only one hand, you can do it. Just be patient with it.
Having said that, the new covers made for the 600 are really nice and look much better than the old 505 cover. They are too pricey, but they do look great.