First of all, home tests are your friend, you must take them.

Personally, I researched, weighed the risks and decided I'd substitute if it came to it.

I suppose you could practice diluting and taking home tests. If you dilute and pass you're probably good but you wouldn't know if you passed due to dilution or it's out of your system. Testing undiluted would answer that.

Going in with known good urine puts the risk at the logistics of the transfer and you can practice that until you're comfortable you've got it right. With dilution, unless you've tested it, you don't know if your going in clean and that is real risk. If you aren't passing home tests I think you should seriously consider substitution.