That's a lot of heat, even for just one minute. The immersible elements for 5kW/220 V water heaters are about 20 ohms (two elements per appliance, Vsquared/R = 2500W).
So your high-power resistor is an item that can be found two at a time in old scrapped hot water heaters. Any plumber can unbolt a few before his next visit to a recycler. A flow of water, or a bucket of water, should cool it well enough.