I have used Certo, not so much Sure-Jell although my friend used Sure-Jell once after getting put on probahtion and worried about going back to jail. He was hooked on Chronic and Vicodin and all sorta crazy stuff, so when it came to the day he drank down that Sure-Jell, whole damn packet, mixed in a little Gatorade and away he go!
They piss tested him and it came back too diluted so they said to come back again next day. Same thing happened so he was arrested and piss tested in the jail by some nazi pigs and this time after few hours the piss test was less diluted and he failed.
So I guess if you worried about them thinking it is diluted urine then maybe it won't work, but it could be worth a try if you have no other options to get outta this situation, then what have you got to lose except a few bucks?