3d selection grid

I know there are an abundance of resources on 2d selections boxes, and I’ve used them in the past.

For my game, I think a 3d selection box would be a lot more accurate. Here’s an example of what I mean (look at 1:00-1:10) :

At the moment, my game is completely flat so I don't need to worry about height differences just yet (so I guess it's still technically a 2d selection box!). I'm using a strict grid for building placement, object placement, and pathfinding. The center of each grid cell is a whole vector3 (i.e. 5, 0, 10) and the size of each cell is 1 Unity meter squared.

Here’s a script I made that instantiates a cube on grid cells according to mouse movement. Lame, but it’s a start. The cube or 'selectionTarget- contains a script that sends information about the types of objects it’s colliding with.

using UnityEngine;
using System.Collections;

public class SelectionManager : MonoBehaviour
    public Vector3 roundedMousePosition; 
    public GameObject selectionTarget; 

    private void Update()

    private void SelectLocation()
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hitInfo;
        if (Physics.Raycast(ray, out hitInfo, Mathf.Infinity))
                int x = Mathf.FloorToInt(hitInfo.point.x);
                int z = Mathf.FloorToInt(hitInfo.point.z);
                roundedMousePosition.x = x;
                roundedMousePosition.z = z;
                roundedMousePosition.y = 0;

                selectionTarget.transform.position = roundedMousePosition;

I need to find a way to click and drag that cube so it can encompass more than one cell at once… otherwise clearing land will be a very tedious process!!!

After some experimentation, I found out that the answer was making a procedural mesh. I altered the code in these tutorials (Unity 3d: Procedural Mesh Generation [SimCity Roads] - Part 1 - YouTube) to fit my purposes and now I’m well on my way to a robust solution.

If anyone is reading this and is interested, reply to this post and I’ll share the code.