One day when your better half is angry with you anyway because you're "working on that damn little car again", steal her Clorox Bleach bottle. The big gallon size. Empty it and cut it in half about 2" below the handle.
Use a hot knife or a razor blade. Voila, the top half is a funnel. Nice and big for oil changes, handle and all, and it fits the oil filler perfectly. Use the bottom half for storing junk in your trunk, the excess chlorine bleach, oily rags, etc.
If you don't have a better half or don't want to cause further trouble, hop down to the local laundromat and scavenge for one. Get all you can, any size. They come in handy for quick oil top-ups and any other liquids you may want to carry in the trunk. I find the fabric covered elastic cords good for keeping them in place.
Author: G. Westerveil