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.