Cystocentesis, and it is the most sterile way to collect a urine sample. They insert a needle directly into the bladder and draw up some urine. It is not without risks though, which is why most vets stick with trying to get a sample the natural way.
I've used the same method as you to get a feline urine sample in clinic, minus the saran wrap. If all your vet is interested in is crystals then sterility isn't as big a deal.
"He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion."