Originally Posted by rebeltx
you should be able to use any funnel that is long and skinny enough to fit in the gas filler tube far enough to push the metal tab and seal open.
If you look closely, what you have are two small tabs on opposite sides of the filler neck, above the spring-loaded flap. If those two tabs are not pushed outward by a reasonably round, proper diameter fuel filler, or similarly sized funnel, then the spring-loaded flap is locked shut as a vapor stop.
So, you can use a funnel roughly the same diameter and roundness as a gas pump fuel filler, like the funnel Ford gives you in the trunk, or get a similar sized funnel somewhere else. But you can't (easily) use just any old funnel.
If you are desperate, it is possible to use some 'tools' to depress both tabs, then push on the flap, and wedge it open with something like a long thin rod to keep it from coming all the way back up and locking. Then, you could insert a smallish funnel. Again, this is last ditch, stuck in the middle of nowhere stuff.