I've got a different one for you. A well washed out 4ltr milk carton. Turn it on its side and cut the top edge of it (infront of the handle). Keep the handle and the mouth of the bottle (both come in handy). Then when your dog squats to pee you slide it where it needs to go, keep holding the handle. Have another container handy (jam jar/urine sample pot) to store the wee in and pour using the bottle mouth of the milk bottle.
It works trust me, saves you getting wee all over you when you try pouring it out of the pan (unless you plan on taking the pan to the vet of course).
Oh and I would maybe do this in the garden rather than the local park