Unable to "cap" a split shape

Hello all

i obtained this shape by splitting with a surface in rhino. There was no grasshopper involved. however i can not “cap” it with low tolerance. I want a closed object but cant get it. Its probably very simple but i jsut cant obtain it.

snow_shape.3dm (3.3 MB)

You can achieve it via Patch and BooleanDifference, and for your reference.