Dartboard Level with a hole to pin it to the board with a dart to free up your hands to turn the board.  It fits perfectly in my Blade 6, but will work on any board by wedging it in the 20 and pinning it with a dart.

Has a slot that will fit a 8x35mm cylindrical bubble level that can be picked up on amazon.  The fit will be tight, you may need to pass the end of the bubble level over some sand paper a few times to get it to pop in.  A smaller level could be used if it is glued in. 

The top is flat if you wanted to set a small level on it as well.