This is a paid asset, but now you can download Boxed Convex Collider for FREE, but please remember this package is provided only for learning purposes or to be able to test before buying the product, NOT FOR COMMERCIAL PURPOSES.
Detail this asset from Unity Store: Original Link
Boxed Convex Collider v1.0
Converts convex Mesh Colliders into multiple box shaped Mesh Colliders to mimic the mesh. The script component can be added to single gameobjects or a single gameobject can be assigned to an editor window to calculate all child gameobjects with meshes. Can lower the Mesh Collider count after calculate to increase performance. Fast, easy and accurate to use.
Converts convex Mesh Colliders into multiple box shaped Mesh Colliders to mimic the mesh. The script component can be added to single gameobjects or a single gameobject can be assigned to an editor window to calculate all child gameobjects with meshes. Can lower the Mesh Collider count after calculate to increase performance. Fast, easy and accurate to use.