These vents do not have to be large at all.
I first will cut one with a razor blade.
You hit the cav. where the short is.
Come out at 1/8 inch the turn a" z ", then make shure the vent leads to open air not a place where the mold fills in when you pour.
The lead will never pass the change in direction, but the trapped expanding air will.
If you make it right it will never even need to be trimmed of at the vent.
Only do one face.
Start tiny and work up to small.
You can make the vent as large as you like after the "z".
Use needle files......dremel can get away on you.
Use the triangle file and draw it away from the cav.
Files only cut in one direction so rubbing it back and forth is useless.
Work tiny to small.
Start at the cav. and work to out side.
Heck draw it on with a pencil for a pattern to follow.
Also this should all be done on a cold mold.
When warm you will rase the edges, and cause flash.