3. Get a fuel-transfer pump system that can transfer fuel from a car fuel
tank to the airplane, that runs on 12V from a cigarette lighter plug or
clips to the car battery terminals.

They make some 12v pumps for the tanks farmers put in the back of their
trucks. I'll bet you could modify one of those.

Any other ideas?

Have you ever seen the calapsable 5 gallon water bladders that campers use?
Maybe you could find something like that for gasoline. Or, maybe you could
call a tow truck. Some of them carry gas. You might be able to talk them
into bringing you some.