Synthetic urine would be your best bet.
If you want to try and get clean naturally.
Eat a high fiber diet with some metamucil.
Exercise as much as you can.
3 days before UA switch to a red meat, high fat diet and stop exercising.
Morning of the UA drink about 40 ounces of gatorade
Pee 3 or 4 times
Use your midstream urine for the UA.
Pickup some cheap thc test strips and test yourself at home.
You can get test strips for about $1.30 each on ebay