On any water holding rectangle or square the pressure with be strongest 1/3 and in the middle of the walls. Something I learned reseaching my rimless DIY build last year. i would build my box from 2x4"s building in support @ 1/3 from the bottom and 2/3 up. Then line the inside with 3/4" plywood using wood glue on edges and all wood to wood contact areas. Then skin the outside with 3/4" plywood. Screw the box together.
You could line it with a pond liner or saving an old swimming pool liner on trash day. There's also rubber tank, research Plywood tanks for brand names. Thatr is basically what your building Minus the window.
Material estimates $125 to $150 for the build without a liner. Two sheets of 3/4" your at $90 already plywood.
Maybe consider a Stock tank from a farm supply most are at least 100gl. Then just burry it or build around it with landscape timber of stone blocks.
OR just do this: