Take one of the original screws down to the local hardware store and find one that matches.
Personally I wouldn't be too worried about leaving the BOV adapter on while you get an oil change. The grease monkeys who do those fluid changes don't give a crap about your BOV adapter. Now if you were taking the car in for some warranty work on the motor that would be a different story...
2016 Ecoboost Mustang
2009 C6ZO6 - Katech Stage 2 #115
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)