I've posted a script on this question which can remove the bottom faces from a MeshCollider at runtime. You can specify an angle limit

The Unity plane primitive has a one-sided collider: only the visible side generates collisions. You can use any object as the platform - just remove its original collider (if any), then create a plane (Hierarchy view, Create/Plane) and adjust its dimensions and position to be the platform "ground". Finally, child the plane to the platform - you can even remove its Mesh Renderer to make it invisible (the collider is independent of the renderer).

EDITED: A simple and seamless alternative is to have a trigger childed to the platform and placed above it, like below:

In the Inspector, set isTrigger in both (the parent platform and the trigger). When the player enters this trigger, set the platform collider isTrigger to false; when the player exits the trigger, set the platform collider isTrigger to true (script attached to the childed trigger):

function OnTriggerEnter(other: Collider){
  if (other.tag == "Player"){
    transform.parent.collider.isTrigger = false; 
  }
}

function OnTriggerExit(other: Collider){
  if (other.tag == "Player"){
    transform.parent.collider.isTrigger = true;
  }
}