Hi,
When I decimated the convex hull to get below the 256 vertices limit, I ended up getting something like this:
A star-shaped base and top 
with a little “dent” on the right side (X>0).
That looks king of cool but to flatten the base I couldn’t select an edge loop or I would have ended up with something like this:
Which seems logical given the shape of the object 
But I would have loved to be able to select a star thanks to the edge loop…
So I selected all vertices, the closest to the middle of the base (with the circle select tool) to set them at Z=0. Now my base is flat 
Cheers !