Safety considerations designing a mains powered device
If the device is properly earthed then use a fuse. This is the tried and tested way. Proper earthing is important but if this cannot be achieved then an RCD doesn't cost very much. To be honest I'd be thinking of a low voltage heater that wouldn't be regarded as a health hazard - maybe 50V or thereabouts. Cost should not influence safety unreasonably and a 1kW AC to DC converter isn't beyond the reach of most folk.

Having said that, if you are considering an isolation transformer, then surely one can be foundf that isolates and drops the voltage to something reasonably safe.