At the cheap / DIY end, you might be surprised just how effective a metal toolbox is, with the paint filed off the seams around the lid, and some aluminum foil to help make connections / cover the gaps to the lid etc.

We had this problem with a test station for Bluetooth Low Energy (BLE) devices in our factory. With over 250 visible BLE devices visible, the local Bluetooth stack would crash. With a USB Bluetooth module within the toolbox, and the device under test also in the toolbox, we could reliably get it to pick up just the one device, so long as the lid was closed and latched firmly. And then confirm with RSSI that you’re only seeing a very-nearby transmitter. :)