Use a convex collider from the mesh.

This means that if you have this set to true, your mesh collider wont have Holes or entrances. Convex meshes can collide with other convex colliders and non-convex meshes. Thus convex mesh colliders are suitable on rigidibodies, if you really need more detailed colliders than what the primitive colliders provide you with.

	transform.collider.convex = true;

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        transform.collider.convex = true;
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		transform.collider.convex = true