If you absolutely have to, you can settle for a screen top. It doesn't filter out all of the UVB, just most of it. And you can take your turtles outside to even it out.
Where have you looked for egg crate?
Oh!! I used some of those plastic bookcase things that you can get from target, the ones you snap together. Let me see if I can find a pic...
I constructed a top out of them and secured it to the tank. It should work until you can find egg crate.