The best way to create mesh colliders for an imported 3d asset?

I have imported a obj file from fusion 360 into unity, its shape is similar to a glass so it is not a convex shape, I need other rigid body to go in and out of the glass. .
If I’m choosing the “generate colliders” option on unity, the mesh is way more complex and contains the maximum of polygons it can, and also it generates a mesh on the top of the glass, so it’s kind of blocking other objects from going in and interacting with the object.
I’ve seen on YouTube some people trying to make a mesh through Bleneder, but I still don’t understand how it works or what is the idle way to to create colliders to an imported 3D asset.

Can anyone help??

Thanks in advance,

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