Just signing in as another NZ member
... Samsara is a Mana (Cook Strait) boat. I am not aware of alloy tanks in NZ different from the Burnsco range. We have the 5kg alloy, but as space is an issue to you could one of the many similarly-sized steel tanks do the job.
A few kilos weight does not seem much of an issue on a 48' boat. Is corrosion
the issue, or are steel versions the wrong size too? Pretty sure I have seen steel 4.5s ... perhaps try Mitre 10, Placemakers and Bunnings ...
By the time you price
in a ten year tank test, a high purchase
price and do an NPV, your accountant would say "throw away a steel tank every few years". But owners of lightweights (me) are just not that rational!