Heavy-duty Key Fob Faraday Box for Surfers

This is a solution to the problem of keeping electronic car key fobs safe while doing water sports like surfing. You lock and unlock the car doors using the physical key incorporated into most electronic fobs. The fob itself is placed inside a Faraday box or pouch and left inside the car. Because the Faraday box blocks the fob's radio signals, the car remains safely locked, despite the presence of the key fob.

A DIY Faraday pouch can be made of aluminum foil, but even a small tear in the foil will render the pouch useless. Commercially available pouches exist, but online reviews suggest these are not terribly reliable. Another DIY solution is an old biscuit tin, but again, people report mixed results with these--the metal is generally thin, the lid is loose-fitting, and if there is a layer of decorative paint, it may provide a pathway for radio signals to leak through.

The solution described here employs Schedule 40 steel pipe fittings, with fine-threaded joints providing plenty of metal-on-metal contact to block radio energy. At almost 4 mm in thickness, the metal is vastly thicker than it needs to be, so the finished product is heavy, but highly signal-proof.

1. 1.5-inch diameter Schedule 40 steel nipple, 4 inches long. This is just a short length of pipe with threaded ends (and no, I don't know why it's called a "nipple"; you'll have to ask a plumber). A 3-inch nipple may be long enough, depending on the length of your key fob. Also check that your key fob is not wider than 41 mm (1.61"), as this is the inside diameter of 1.5-inch nipples. It's unlikely you'll find this at your local hardware store; you'll probably have to go to a specialised plumbing supply shop.
2. Two steel end caps.
3. Two scraps of foam, ~1 cm thick, the size of the end caps.
4. Gorilla glue.
5. Cardboard toilet roll.
6. Sharp knife or scissors.

Cut two pieces of foam to protect the fob from being damaged by the metal end caps.

I used some scraps of polyurethane foam I had, and found I could cut it to shape using the nipple as a cookie cutter.

Use the Gorilla glue to secure the foam discs inside the end caps.

This will also help to protect the key fob from damage.

Use a knife to cut a lengthwise slit in the toilet roll. Then insert the cardboard into the nipple. By luck, a toilet roll turns out to be the perfect length for a 4-inch nipple!

Don't be tempted to use Teflon tape on the threads, the way you would for water or gas pipes. Anything that reduces metal-to-metal contact creates the possibility of signal escaping. You should probably even wipe away as much manufacturing oil from the nipple and the endcaps as you can, for the same reason.

To the untrained eye, this DIY Faraday box looks a lot like a pipe bomb. You don't want to be responsible for causing a bomb scare if this thing gets stolen and discarded near a public building, so put your mobile number and/or email address on a piece of paper and secure it to the nipple with clear tape.