If you build a structure out of supermarket trolleys around the car, would that be sufficient? I love the idea of a car size foldable Faraday cage btw.
Serious answer? Maybe. It would depend on the frequency the keyfob is using, and the spacing of the grid, and the metal they're made of.
There's a good chance a cage made out of shopping carts would block HF and below quite effectively. UHF and microwave will go straight through it though. Any holes need to be significantly smaller than the wavelength.