FPS Object-on-wall Placement

I want to build an FPS adventure game that lets the player instantiate objects on the wall of a cave (example: a torch) but I don't know how the best way to do it would be. I have a nice cross-hair and raycast to go with it but it is the object instantiation that is puzzling me.

I have a few ideas but I'm at a loss for where to begin:

1. My first thought is a minecraft-like approach with specific areas with grids that only allow torch placement in specific areas.

2. Free-form approach that allows the player to place a torch on any valid surface in any location.

3. Pre-placed torch areas that instantiate a torch if the player clicks on them. (Leaves the least amount of freedom for the player).

My ideal option would be #2 because it gives the player the most control over their lighting but I could also see it looking really bad if done improperly.

Here is my raycast code (its only slightly modified from the one in the unity documentation):

public class centerCameraRaycast : MonoBehaviour {

 public Camera mainCamera;

 void Update() {
     Ray ray = mainCamera.ViewportPointToRay(new Vector3(0.5F, 0.5F, 0));
     RaycastHit hit;
     if (Physics.Raycast(ray, out hit, 1f))
         print("I'm looking at " + hit.transform.name);
     else
         print("I'm looking at nothing!");
 }

}