support 9 1500 watt electic watter heater elements in a circle at 45 degree angles to one another... mount an aluminim plat on top and on bottom conneted between one another to transfer heat between the two think aluminum plates... and support the burner so it will hold a 6 gallon stock pot... mount assemablly onto a mettle plat so the heat transfers properly and doesnt start your floor or back yard on fire... then attack a ring of aluminum rods around or thin strips or a solid rim that would be about half way up your brew pot (will increase heat transfer)... that will be 13.5 KW burner, and get you decent heat transfer efficenty... say 85%??? so 11kw... or just short of 40,000 BTUH??? that should boil your water in about 15 minutes... you could control tempature by using seprate toggle switchs to turn one or off/on individual burners. Have a main disconect for total power feed... only problem with this is that youd need a well over 120amp circut 220 to run the damn thing and each burner would cost 20-30 dallors and the aluminum will cost a pretty peeny... youll also need to weld to assemble it... so this isnt simple....
heres a better idea Take an old water heater burner setup, mount it on a grill stand, cover it with a salvaged aluminum water heating plate in the upper part of the flames... you'll need to use Natral gas but that will give you a 40,000 BTU burner with about 70% efficenty, say 28,000 BTUH... that will take longer but will be easier and cheaper to run... using propane will cut that in half and take twice as long and youll get a soot build up on the bottom of the plate and get worse heat transfer... Simply change the oriface with a larger one (or simply drill it out a little bit) and that will increase your burrner output and thus youll boil faster.... thats the best i can think of...