Yes and no. The biggest problem you'll have is sealing the box completely. Aluminium tape should solve this issue, as would wrapping aluminium foil around the key or tin. Testing to see if it works is very simple: stand next to your car to make sure it opens when you're near. Then lock it once, put the keys in the tin and try opening it again. If it stays locked then your tin is acting as a Faraday Cage. Though if you have to tape and untape it every time you want your keys then it might be worth spending £7 on a Faraday Bag that you can just open and close...