I would use the idea but use ABS sheet, much tougher and less risk of shattering or cracking. Can be glued together. CT1 would be ideal as both sealer and joining – bit like silicone but much stronger.  Or plan B, get hold of an old copper tank, cut it up and soft solder what you need. Noel.

Perfectly viable approach.

I fabricated a fairly complicated motor-shield / splash-back for my Myford lathe, from 3mm PVC sheet.

I used standard plumbing-pipe solvent for the joints, but reinforced the corners with “angle-plastic” made by forming strips over the edge of a length of planed wood, softening the PVC with an electrician’s heat-gun.

No “welding”.

To improve air flow round the end of the motor I dished the adjacent panel by pressing the heat-softened plastic between two improvised plywood dies whose diameters happened to be suitable, using the bench-drill as an arbor-press.

(And no I would not use the drill as an arbor-press otherwise!)

Some builders do use mild-steel for water-tanks, and coat their interiors with petrol-tank sealant.

An alternative might be to make the tank of steel with a screwed-on top plate (with rubber sealing strip), so the walls can be coated internally with fibre-glass and resin, using the kits sold for minor car-body repairs.