My wife has purchased disposable travel urinals before, but they cost $1.50 to $2 each. This is cheaper.
Step 1: Supplies
1 disposable cup (I use old ones destined for the garbage)
1 plastic newspaper sleeve (again re-used). Alternatively you could use the little baggies made for dog poop.
1 diaper booster pad, or bladder control pad. Alternatively, you could repurpose unused diapers your child has outgrown
tape (duct or shipping tape)
Step 3: Make a Container
If you've got one of the dog waste bags instead, you've got more room to work with. You can attach the pad to the outside of the bag using the sticky backing of the pad, then turn the bag inside out.
If you're using an old unused diaper, cut out the middle absorbant part of the diaper and use that.