The problem with standard immersion elements is that they are made from nickel plated brass, so any fracture in the nickel plate leads to a very fast erosion of the underlying brass. If the base is flat and stainless steel, simply take a flat plate stove element, clean it down to bare steel and use a thin layer of silver solder paste to bond it to the base of the pan. You use the element to heat the silver solder up to melting, but it will only have to do this once, as normally a lead load will draw enough heat to prevent it melting again. The element will probably survive the silver solder at least once, as they run at red heat in cooking with some settings. You get silver solder paste used for preform brazing of stainless steels, which have a rather agressive flux carrier, and which only melt at over 500C, so are perfect for this. The standard use id for brazing stainless steel mesh or for bonding sheet edges in industry. will probably melt at red heat, but as long as you clamp the pot firmly to the element and only rely on the braze for heat transfer it should work for a few dozen cycles. If it fails at least you will be able to undo it with a gas torch.