Ok man, I had a dt just last week and I was freaking out about it until I found out how to sub quite easily. You don't need expensive kits like the Urinator or anything like that.
First thing. Since it's a pre-employment test, they will NOT watch you. They can only observe you for probation tests/court-ordered tests. So you don't need anything elaborate. Just a way to smuggle clean urine in there while keeping it warm.
I had a friend piss in a clean, 3 oz travel-size bottle of shampoo that I bought from Target for like 75 cents. Test wasn't for a few days, so I put it in a freezer to keep it fresh.
Day of the test, I took it out of the freezer an hour before I left. I heated up some water in a bowl, then submerged the frozen piss into the hot water. DO NOT MICROWAVE HUMAN URINE!!!!!!! You must heat it up the way I did.
Get yourself a thermometer and check the piss temperature once it warms up. Let it get above 100 F. Next, you're going to put it beneath your nutsack to keep it at body temperature. This is very important. Once it's over 100F, put it under your sack to keep it warm. I wore two pairs of tight-whities plus flannel-lined jeans just to insulate it further. Plus, the tighty-whities helped keeo it from slipping down my leg. If you are paranoid about the temp, you can buy some handwarmers to keep around the bottle while it's in your pants, but I found that my body kept it perfectly at 98 F without additional heat sources.
Once you're in there, they may or may not make you wait about 45 minutes before your test (they made me wait). I believe this is an anti-cheating procedure, but I just sat in the waiting room, legs squeezed together, keeping the bottle warm.
They will make you empty out your pockets, but they can't legally search you or pat you down. The bottle being hidden in your crotch is perfect--they will never find it.
Go into the bathroom, don't flush the toilet, and they will tell you to put the cup on the back of the toilet. Wait until they leave, lock the door, and pour the contents into the cup. Peeing in the toilet is not necessary so I didn't do it.
3oz is a perfect size because the most they will ask for is 60ml which is ~2.02 oz. I've heard horror stories of people who smuggled in a 2oz bottle only to have their specimen rejected because there was not enough pee; they had to wait and give another sample, but they had no more clean urine, so it was either use their own urine and fail, or refuse to give another sample, which is considered failing. Don't let this happen to you.