Thanks for all your suggestions. I tried the plastic bag with a diaper over it. It would have worked had it not been for the sample running down her legs.
So, I resorted to holding her for four hours and locking her in the bathroom with a piddle pad on the reverse side.
Worked like a charm even though she thought she was a bad girl doing it. Poor thing...So confusing for her. I took the sample to the Veterinarian's office yesterday and now she and I both are waiting for the results. Thanks again for all your help!