I use the same method as Cinder. I had to get seven samples at once.
I use a small lid, you can also try a baggie. Hopefully your litter box is large enough and has low enough sides so you can get the lid under the cat without disturbing too much. ( remember they pee slightly backwards, not straight down so you have to be pretty careful when placing the catch container. You will get peed on
Having done this for a few years I have discovered my cats like to pee between 3 and 5 am for some reason! So I lock all of us in the living room, put the favorite litter box right in from of me laying on the couch.
sometimes cleaning the box and adding new litter then putting kitty in there will get them to go as well. My crew can't resist dirtying a nice fresh box immediately.
You don't want any litter particles, or cleaning detergents in whatever you catch and store the urine in. These will cause false results, (usually false positives)
Ok, one more thing about specimen handling. Urinary pH can change very quickly. The sample should be refriderated and kept out of the light until you are ready to head to the vets office.
Good luck! It isn't easy, but is possible.