Makes sense to me. Exactly what I’d do if I ran a coffee shop (not that I’d do any such thing). I’d also put in cell-phone jammers. Oh, they’re illegal. Well, a Faraday cage, anyway.
You might have a winning business model there. Advertise the coffee shop as a shelter from EMPs and solar flares. Put in shelves by the door, where patrons can park their tinfoil hats, while in your sanctuary.