You have a point. But a full cage is probably unnecessary for some devices. Back when I worked on things like this we had a bucket. It was the 'out of coverage' test metal bucket. You had to be careful to get the whole wire harness inside the bucket or it still could sneak in there (as cell coverage is sneaky). I think someone picked it up at local Tractor Supply chain. Someone else had found a small decorative bucket that was meant for plants that just happened to fit perfectly over our smaller IoT devices. It was maybe 5 bucks of materials. Also you might be able to get away with a simple shed that is made out of tin. Havent tried it so your mileage may vary. Retrofitting a house would probably mean taking down the existing exterior or interior walls. The materials would not be that expensive. Most of your cost would goto hiring someone to do it right.

The big downside to my approach is heat dissipation and servicing.