i got the ones i have out of cylinders that i scrapped - and i think they must have been in the top (else they wouldnt have fitted, i guess!)
my friends reactor is a copper tank, upside down, so the element would fit, but its only a small reactor (140ltr tank. i think) so im concerned about having the element going so high up the tank - leaves less room for error.
(yep, i know a level switch should probably be fitted, and i offered him one....)