[Gelbster]

I fabricate aluminum and brass. I have had one Boxster plastic tank failure and a junk generic replacement.I thought about making a replacement for a nano second because I am slow .My limited skills tell me this is not a practical project. The tank is easy.The fittings would be very,very difficult.They are not generic so the fabricator can not just braze on a stock fitting to a custom tank - that is the usual method. If you are thinking about cutting off the existing plastic fittings and gluing them on to a custom aluminum or brass tank ..... cough, that won't last very long.
Some other manufacturers have the same problem and their tanks are similar price e.g. Land Rover.