look at the total length and width of your unfolded tank as shown in the picture.
compare that the the size of the sheet of material you have.
That gets you 2 constraint equations.
You use 2 Lagrange multipliers and solve the system of 5 equations.
P = vol - a*length_constraint - b*width_constraint
dP/dh=0, dP/dl=0, dP/dw=0, dP/da=0, dP/db=0