NavList:

Essentially none of this has been tested. Faraday cages are real things, of course, but nearly all of the advice that you're seeing here in these NavList messages and elsewhere on the Internet is just lore. Some people recommend carefully sealed, solid metal cases. Some insist that the metal has to be of some thickness and the cage of some size. Others contend that an actual "cage" --an open metal mesh, like a birdcage-- is sufficient. Grounded? Not grounded? Will a microwave oven work as a Faraday cage (it should)?

There are facilities which can generate "artificial lightning", and these are used to test hardened electronics, but none of the homebrew Faraday cage arrangements have been tested side-by-side, as far as I have been able to determine. What do you really need in practice to protect a common scientific calculator? Do you need better protection for a more sophisticated device like a smartphone or tablet? I expect so. What works? What doesn't?

Lightning strikes that damage or fry eletronics on boats are not rare, but the vast majority occur dockside, at marinas and in port. We hear anecdotal evidence when owners return to their boats usually days or weeks after the supposed lightning strike, but there's no way to be sure what really happened. Perhaps we need to commission a vessel to sail out into some thunderstorms loaded up with a few ddozen variants on the Faraday cage concept to find out what really works. 

Frank Reed