Given your history I think subbing is going to be your most reliable method. It could be another month before you piss clean.I would also recommend just going the synthetic urine route. More reliable.
It will stay up to temp for quite a while(hours) when under your balls.But its better to have it a little warm rather that a little to cold. You can always blow on it to get the temp down.
They won't be grabbing your crotch. They'll just ask you to empty your pockets.