Try Tek Tanks for a made to measure option and C.A.K. Tanks may have an off the shelf tank that fits your boat.......if not they too can knock you one up I think! Beyond that have a look locally for plastic fabricators who will do the same!
Not really suitable for a boat - wrong material (by the looks of it) wrong shape, no baffles, not inspection hatches. A properly made tank from somebody like Tek Tanks, or a metal one custom built is a completely different animal.
To re-quote my post "I had one made to measure by a guy I found on eBay. Not necessarily this one but similar" the exact shape I wanted, with baffles, Inspection hatch, inlet, outlet and gauge hole. The plastic was suitable for potable water and was plenty solid enough although it has enough support where I installed it. I don't know how you can tell it isn't "properly made" as both tanks from a branded company and from an individual engineer are made by humans with sufficient skills to do the job. Plastic vs metal is an entirely different discussion, and having had metal tanks fail in the past I'm a convert to plastic. If you want to pay a premium to cover the overheads of a manufacturing company then please go ahead, but there is no cause to rubbish the work of a private individual without even seeing it.
What you describe is nothing like the one in your link, so not sure why you posted that because it as I described and not suitable for a boat - nor claimed to be. Pretty sure if you had posted a photo and details of your actual tank I would not have made the same comments.
