I'm picturing trying to push slivers of saran wrap past the fuel fill thingy. This is a great idea but needs a delivery system.
I'm picturing a plastic / rubber tube, maybe a foot long, plugged at one end with a marble, then filled with the slivers and finally a wooden dowel sized to slip through the tube with some resistance (you don't want it to fall out OR be too hard to push through).
Insert he marble plugged end into the fuel fill, push the dowel (like a hypodermic needle) and the marble and slivers will be pushed straight into the tank.
The only issue is those fuel doors that are keyed or are released with a lever from inside the vehicle.
Disclaimer: I would never do this and would discourage anyone else from trying. Just thinking "out loud". I like solving puzzles!
Just fill a proper sized hose with the plastic pieces, insert in filler tube & blow. As far as a locked cap........puncture tank....toss match....done.