I think having the board level is about the best you can do.
Maybe incline it slightly nose down so the nose is not significantly higher than the vent fitting.
The water vapor is giong to "flow" toward the lowest pressure area and that should be at the vent fitting.
As far as the O'ring damage, that's what happens to O'rings when you "squash" them flat. Not what they were designed to do at all.
Maybe don't tighten the plug so tight so you don't "squash" the O'ring
so much that it cracks or splits.
Here's a link to how an O'ring is supposed to be used as a static face seal:
Since we are using a screw thread in the middle to apply the compression to the O'ring, the groove would need to be up inside the head of the plug.
After market plugs with the correct thread size would be wonderful, but I've never been able to find one.
Hope this helps,