I think your best bet would be to substitute with Quick Fix synthetic urine. Eleven days may not be enough time for you to get clean even with your body weight. Most pre-employment drug screens are unsupervised so it should be a piece of cake. If your not comfortable carrying this in then dilution is your next best option. Read both the dilution and substitution stickies at the top of the forum, they have all the information you need.

Don't waste your money on detox kits, they are unreliable and expensive.