I have had two cats in the past who formed crystals in their urine, and I was often having to do urine samples. I could never get myself to be a fan of the vet version, where they put a needle though their abdomen and into their bladder to withdraw one. OUCH!
My cats would never pee in an empty cat box - they were much too polite to use something that did not look to them like they should be "going" there. So I got creative.
Take a clean cat box, and fill it with those shreds the you buy for gift bags and easter baskets - clear works best but yellow will work too. My cats thought I had changed to some really weird cat little, but would use it. The urine sifts right through. Move it out the the way, suck up the uring in a syringe, and voila! A perfect urine sample!
Hope this helps someone.