You may use thin thickness for the wall if you install external stiffening frames. The use of stiffners is the most popular design practice for rectangular tanks in internal full water or vacuum condition.
Please find the following references;
1. Eugene F. Megyesy, Pressure Vessel Handbook (Chapter Rectangular Tanks)
2. Warren C. Young, Roark`s Formulas for Stress and Strain
Please find the following references;
1. Eugene F. Megyesy, Pressure Vessel Handbook (Chapter Rectangular Tanks)
2. Warren C. Young, Roark`s Formulas for Stress and Strain
Thomas Eun