Subbing sounds like its going to be your best option. They will not observe and 2 ounces is all thats required at Qwest and Labcorp facilities. Keep the temp in range and you should be fine as long as you're sure the donor takes NO drugs of any kind.
Dillution is probably not an option with you but its always a good idea to do it the morning of the UA just in case anyway.
If you heavily dillute your urine you can use a little bit in the sample but only if its an emergency.