Doesn't that fill plug have a simple pipe thread? I believe a vent might be a good idea to prevent pressure buildup inside the box from pushing lubricant past the seals when the under hood temperature gets high, but I'd suggest drilling intersecting holes at right angles, with the hole to the atmosphere coming in from the side.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!