Yeah it’s custom, it has a step up , a fill port in one corner, and angle brackets on the top to hold a pump. Sizing is odd because it fits down in between some other stuff

Do they need it to be a custom size?  If not, You could probably find a aluminum tank from a semi truck salvage yard. They come in all volumes from 50-200 gallons and are already made to hold fluid.

A family member has asked me to build an aluminum tank to fit on the back of an amphibious vehicle for his fire dept. Its not large, total volume is about 90 gallons but needs to stand up to normal wear and tear. Does anyone have a gauge of what material thickness they'd recommend and what alloy?

Aluminum Tank for Fire Dept...thickness? Alloys?