As I envision your description and knowing that an Allen set screw has a hexagon key hole part way thru it*, you first measure the depth of the key hole.
You then add 1/16" to this value and file the end opposite the key hole to end up with an overall length that is the same as this larger value.
This would be followed by drilling a 1/16" hole thru the remaining wall at the bottom of the Allen key hole.
Sound right?
* a 1/4-28 UNF set screw has a 1/8" hexagon drive, 1/8 minimum deep.
If the set screw drive hole is 5/32 (.1563) deep, that plus 1/16" (.0625) wall thickness would result in a vent plug that is .1563 + .0625 = .2188 or 7/32 long.
You then add 1/16" to this value and file the end opposite the key hole to end up with an overall length that is the same as this larger value.
This would be followed by drilling a 1/16" hole thru the remaining wall at the bottom of the Allen key hole.
Sound right?
* a 1/4-28 UNF set screw has a 1/8" hexagon drive, 1/8 minimum deep.
If the set screw drive hole is 5/32 (.1563) deep, that plus 1/16" (.0625) wall thickness would result in a vent plug that is .1563 + .0625 = .2188 or 7/32 long.