My recommendation is to go for a plastic tank, not the stainless steel as used in my 2006 331.
I have this summer had to pull the tank out for some welding on one of the hose spigots, crevice corrosion. Siltala apply an adhesive where the hose is clamped and that together with ‘poo’ sticking on the inside leads to corrosion. The hoses are a ‘pig’ to remove because of the adhesive !
There are plastic tank manufacturers, e.g. TekTanks, who will make, (at high cost), you a tank to fit a suitable space using dimensions you supply. Whatever you choose to do ensure you have an access hatch fitted to clean the tank out.
Good luck with your project.