I bet there aren't. They'll need the back side completely flat for the heat sink.

You can see some examples in the EU thread:
https://bitcointalk.org/index.php?topic=250249.msg3031271#msg3031271
(although their boards are likely slightly different, but the point is the same - you need it as flat as possible)

Here are some more images:
(check 72 and 73)