I saw an iron lung at a surplus atore once and wish I'd bought it. A thick steel pod like thay would help with low frequency magnetic fields ("ELF" fields), which an EMF tent doesn't block at all.
The cheapest option might be to convert an old fuel tank like this:
But there are potential problems, for example some 55 gallon drums have caulk in the end-seams instead of metal-to-metal contact (which limits their RF blocking performance). And you'd definitely need to clean it up and make a door with continuous contact along the edges, and add mesh vents for fresh air.