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.
Step 4: Attach Receptacle and Container
Step 5: Enjoy
Ladies: Press the cup to your lady areas and go.
By the way, were you disgusted by the first picture showing me using the urinal? HA! Fooled you. I'm not really peeing in this picture. I am squirting water colored with food coloring from a squirt bottle. Don't believe me? Here's a picture proving it.
I emptied a 16oz squirt bottle into this urinal, then turned the whole thing upside down almost immediately. No runs, drips, or errors.
16oz is a pretty healthy amount of pee. I've peed more before, but only after drinking copious amounts of beer, which you shouldn't be doing in a car anyway. So I think this has got you covered.