Stainless steel?? Heavy! If you or someone you know can weld stainless, surely you know somebody who can weld aluminium?
I did some back-of-napkin maths on this, and
actually, I was wrong about the magnitude of the difference...
Let's assume you want a 75mm internal diameter. Perhaps excessive for a stocker, but if you're spending the money on materials, welding, electropolishing/pickling & passivating (for SS), you want to cover all bases for future power mods.
In stainless, you're looking at 76.2x1.6 tube, I'd go for grade 316. That clocks in at 2.95kg/m, and you'll use roughly 2m of it. So, with two elbow joints and a head nozzle, about 7-8kg all up.
That's not too shabby, and you needn't worry about galvanic corrosion.
Conversely, aluminium requires a greater wall thickness of 3mm. The closest match is 80x3 6060 T5 tube. It's 1.96kg/m. So really, you'll only shed about 2.5 kg overall switching from SS to aluminium.
So.... When opting for a custom metal snorkel, go with whatever material is most accessible, or whatever you can fabricate for the least cost.