Have any of you fellas ever used one of those "Flo Thru Manifold" assemblies used for heating spas? I've seen them with 4KW (@240V, 1000W @120V) and 5.5KW (@240V, 1375W @120V) heating elements installed. It would seem to be the perfect pre-assembled HEX. They can be found on eBay for less than $60 shipped if you look around a bit.
Heater tube shell:
Heating element that's inside: