Storing water in a "rectangular bin with a hopper" is unusual and an expensive waste of materials.

However, storing virtually any type of liquid in a cylindrical tank with a cone bottom is very, very common.

Does your boss (or the client) have some type of advanced business management degree......perhaps an MBA ??

Many reasons besides cost would necessitate rectangular or non-circular storage. Just a few - space, transport, available shape...It is client preference not mine.

I don't think there is a "right direction", but several different approaches would work. You could use finite element modeling for the whole thing. You could use flat plate formulas from Roark, I think he has one or two triangular plates in there along with the square and round. If you have stiffeners, you can treat plate between them as beam sections.

Generally, if it's small, you want to use gross approximations and figure the time and material savings aren't worth more effort. If it's large, then worth spending a little more time on.

Thanks JStephen, that is useful to me! The tank is in fact quite small so extra effort is not desired. The biggest approximation I can forsee is treating the edges as being fixed as in reality they may not be fixed in every respect when doing FEA which is what I did.
Thanks again.
Michael