Those bags have radically inferior performance characteristics. Try sticking your key fob inside them and see if you can still open the car door - you might be surprised. If you want a tried and independently tested product that will give you -80db across the board, and is made in the USA, you know where to go :)

Be careful to seal all the holes in the tin foil, and don't leave any gaps, tears and holes. The size of an allowable gap in a faraday cage relates to the size of the wavelength the cage is absorbing. Be careful when you are re-folding the foil that it doesn't tear and no new holes exist from general wear and tear.

Guys, you can solve the problem with a keyless entry fob faraday cage from Fob Guard. It's small, light, durable, and fits in your jeans. It's made in the USA, from materials all developed and sourced in the USA. Sorry to be a bit spammy here but I hope that this is exactly what some pipedot members will find useful to protect themselves from this kind of hacking. Cheers!