Ok. Have it your way. Based on that, life begins 40 days after conception. Because Aquinas says so. Even though later teaching has clearly changed that.
In the same way, the Church does not arbitrarily condemn dressing in costume and participating in theater any longer. A Halloween costume is of that nature, not cross-dressing.
To the OP, wear the costume, have fun. If someone is scandalized by it, that is an issue they should take up with a priest or therapist.