They will not make you strip down naked unless this is a military drug test and physical. Women have a lot better chance of substituting then men because of privacy issues. You said that your 19 days clean and have another 7 till test time? I would pick up some Quick Fix and another home test kit. The day before the test I would check yourself with the first void of the day to see where you stand. If your passing then follow the dilution method to a T the morning of the test. If you feel comfortable with substitution then I would go that way, you should have no problems concealing it. Just make sure you read the substitution sticky and practice getting your temperature right.