If you attach a script to the "hole" object, you could detect other objects entering in OnColliderEnter(). Then, using the OnColliderStay() method, you could move them downwards "through" the hole, then destroy them (or whatever you want to happen when they pass through the hole).

It's kind of a hacky solution, but it would work if you're unable make a custom asset. It's not going to give you physics engine interaction though, so making the object tumble realistically through the hole will be more effort than it's worth.

For modeling and mesh prototyping within unity there are very cool assets, such as this, or this. The most affordable of the solutions of course is to import the model you want as wanted from your modeling program.

It would expected to be possible with pro-builder (modeling tool integrated in unity). However its not.

Window->PackageManager->Probuilder Then you need to enable the experimental features: Edit->Preferences->ProBuilder

Then there is a tool that its supposed to make union, instersection and substraction of meshes:

However it does not work and at least for what I tried, I obtain this error always:

StackOverflowException: The requested operation caused a stack overflow. UnityEngine.ProBuilder.Csg.Plane.SplitPolygon

(Unity version 2018.4.12). Maybe it can be checked if there is an issue with the unity version and if the pro-builder approach migh work in others.