There is no shortcut at all. My idea could be select the inner vertex side of your container, mesh and duplicate it with shiftShift + DD.
Then press P for convert that selected vertex onP to separate the selection into another object. You
You can try to fill it a bitmake new mesh closed by selecting its top loop and pressing FF and with luck get it close without much work (the mesh must be closed forthe Fluid sim to work like fluid).
Then you need assign it like "fluid" on fluid physicsadd Fluid modifier in Physics panel.