I guess that depends on whether you or someone else makes it, how many angles are required and how complicated you need to make the plumbing
You'll have to ask locally.
Another option is to look at the Ronco and Tempo companies to see if your dimensions match a tank they already make. Because the consequences of failure are severe, beyond jugs lashed to the mast
, I don't advocate a DIY approach on tankage, unless you are a fabrication wizard.
You may also consider two 20 gallon tanks
for trim and ease of installation
, although that doubles up the plumbing
. If you get one tank contaminated or dirty, you aren't stuck with your mouth pointing up, waiting for rain.
It's really limited to your own needs.