Greg,
In the early days of cellular phones, a friend had a series of metal sheds, fixed up pretty nice inside, in which he ran a business of taking muscle cars from the 60s and 70s and taking them completely apart, down the nuts and washers, and putting them back together all fixed up better than new.
Quite the operation, and quite the price, but a fun place to stop by and see what he was working on. Anyway, he would lament that with the metal buildings and built as tight as they were, cell phones worked poorly inside. Another common friend who was into electronics from the ground up, did some sort of passive antenna with one outside, connected to another inside which apparently re-radiated the signal and it worked pretty well.
Now I understand that sort of amplified cell antennas exist, with an out of doors antenna and then a smaller one in the house or office to re-radiate. I'd think they'd get RF feedback, like a mic in an auditorium, but they seem to work.
I am fascinated by antennas hand have zero idea how the darned things work, which I probably won't be able to solve before I die.
Nonetheless, I think your comment about any electrically transparent hole big enough to let radiation in or out would do.
I'm wondering what the limit is for size. 1/2 wavelength? 2 wavelengths? Just curious.
Dale