I've actually seen people doing this. I wish I had one too. In few decades we'll have Faradey chambers for rent on per hour basis, just like music studios nowadays.

For smaller rooms with good HVAC ventilation for fresh air, EMF reduction can be achieved with removable clamping ZipWall/FastCap poles to support grounded aluminum radiant barrier on walls, roof & floor. E.g. Attic Foil has white vinyl on one side and conductive foil on the other side, about $0.25/sq ft. Matching tape for joints.

To frame the shielded room-within-a-room, cheaper alternatives to ZipWall/FastCap are PVC furniture pipe, electric metallic tube (EMT) conduit or 2x4 pine. Doorway can be shielded with radiant barrier and RE-U-ZIP magnetic entry strip for a reclosable opening. Bonus side effect is thermal insulation.

Transparent RF shielding for windows is more expensive ($4/sq ft), https://safelivingtechnologies.com/signal-protect/.

Use air quality and EMF meters to validate the design and monitor ongoing effectiveness.