Cut a tank in half and reweld it, if you can weld ali as a professional , you could do it.
But to get it recertified, that will be another matter.

On dump valve, any manualy operated 2/2 valve of the correct pressure rating will do the job nicely. I even used adapted fire extinguiser valves for that job.
But simplest is a hydraulic 1/8 bsp ballvalve.

The dump vales you found for cars are more like the mandatory PRV we need. Pressure Relief Valves. In cars it's used to protect the turbo and inlet manifold from overpressure.
In the robots it's to have a safety feature in the pressurised parts. The PRV has to be set at the max working pressure of the lowest rated component, or at 1000psi, whatever is lowest.