[formerly_proven]

I think the biggest issue with cookie tin cans is that they usually don't close all the way around, because the lid or the box is always slightly oval and not round (they always "stick" just in two or three spots when opening), so it looks like there's a small gap around half the circumference long - far bigger than the wavelengths in question.

Square tin can works better because those seem to usually scratch on the corners. The smallest square tin can you can find should work best. A tin can sealed with adhesive cooper tape should work pretty well even if the copper tape is isolated by the paint on the can.

[mcshicks]

I had to do some testing with ble temp sensors once we're I needed to isolate them and then put them in a freezer. They were in advertising mode so I could tell at least if I could see them with a phone. A round tin that was used for green tea from Japan was the most reliable method I was able to improvise. It fit very snug and the lid was about an inch deep.

[aurizon]

Yes, a one inch compressed gap like that is a tortuous attenuative path in it's own right...