Time to cover walls with tinfoil? :)
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.