Re: Please recommend custom tank makers
Would not recommend any metal tank for holding.
I do recommend Raritan Engineering for plastic holding and water tanks. They have approx 1500 shapes/sizes in their catalog and will make any shape\size you want plus place inlet/outlet/vent/access where ever you stipulate. I ripped out our old fiberglass holding with leaking stainless fittings and had them build a made to measure one, cost ~$400 (a few years back).