Aluminum foil works. At least, I can't receive calls or texts through it last I tried.

Get the heavy duty kind. It's not any more conductive, but is more durable against tearing.

Note that a gap in your phone's tracking data can look suspicious at times. Sometimes it's less suspicious to leave your phone at home.

Bags don’t really work, but hard shell boxes do.

Back when I was working with radio devices, we needed real isolation on lab benches, along with the ability to selectively allow RF paths with specific impedance. The gold standard was Ramsey test equipment enclosures, and they really work (although they only provide isolation up to about 90 dB at the frequencies we cared about; for extreme isolation sometimes we had to nest two like Matryoshka dolls).

It doesn’t sound like you need any conducted signal, just isolation, so that will make for a cheaper bulkhead. Here’s the smallest/cheapest Ramsey enclosure. You can probably find used ones on ebay for less, but you may need to hunt for a while.

Another company that makes real enclosures is ETS Lindgren but they’re larger and much more expensive.

If you don’t like the weight/bulk/cost of that solution, then no, you’re not going to find something that actually works.

Using a few anti-static bags inside one another doesn't block 100% of signals but it cuts range down by a lot. But also, if you're looking for this high of a security level, wouldn't it be easier to just find a phone with a removable battery?

First off, there's 200 more practical things you can do first. A Faraday bag isn't going to accomplish more long term then your phone simply being stripped of bloat and tracking apps, or simply being turned off.

That being said, if you must, Mission Darkness bags are decent, and I've tested them and they do block WiFi, BT, and cellular signals.

YMMV, and understand that when you phone is in the bag, it might as well be turned off. You can't access it without opening the bag.

Are you trying to EMP-proof your phone, or block signals/tracking for a time?

I don't think I am the target audience for these products, and have no answers for you, but am curious what these would do above what turning the device off. Is a device that is powered off still reachable? Or is this not about that?

Anyway, on to the non answers, when i worked around wiring that needed to not be affected by interference from adjacent wiring, it would we shielded by aluminum. It was thicker than household foil, and had braiding. The wires could be run with other, non shielded wires, most of which were dramatically different voltages.

I am not a scientist 😅

SLNT Faraday stuff is supposed to be good. I've never used one. Michael Bazzell recommends them.

Something meant to maintain a high or low temperature. A Thermos or pizza delivery bag or a bag for bringing frozen groceries home. Some might just be foam, but there are also ones with metalic coating or a steel shell.

I have a bag from silent pocket. Works great.

Why would one want to put their phone in a faraday cage? I know what the cage does, but don't understand why you'd want to use one instead of turning the device off.

Edit: According to this, unless your phone is compromised, turning it off is sufficient.

PDF link: https://media.defense.gov/2020/Aug/04/2002469874/-1/-1/0/CSI_LIMITING_LOCATION_DATA_EXPOSURE_FINAL.PDF

From what I heard you won't find a real Faraday bag that works well and blocks almost all signals, those found on Amazon are really not that effective and only "military grade" bags could be useful in these kind of threats

I do!

My use of a Faraday bag is more prepper oriented (i.e. solar flares) instead of geared towards privacy, but I keep a Pixel 8 with GrapheneOS, linux laptop, and eReader in a SLNT Faraday bag.

My tests have been far from perfect, but the phone's Bluetooth and cell reception have all ceased working once put in the bag.

i made one with tinfoil and a ziploc bag