How to set the bounds of movement for one object to the dimensions of another object

Hi - I want to figure out how to do this with scripting and not with a collider object - I have a cube which sits on top of a plane. Using certain keyboard keys it can move around within the scene, but I want to restrict it to the dimensions of the plane it sits on. How would I do this? Here’s my current code (no bounds):

var speed : float = .01;
var newObject : Transform;

function Update() {

	var moveRight = Input.GetKey(KeyCode.X);
	var moveLeft = Input.GetKey(KeyCode.Z);
	var moveBack = Input.GetKey(KeyCode.V);
	var moveForward = Input.GetKey(KeyCode.C);
	if (moveLeft) {
		newObject.Translate(transform.right * -1 * Time.deltaTime * speed);
	} else if (moveRight){
		newObject.Translate(transform.right * 1 * Time.deltaTime * speed);
	} else if (moveBack){
		newObject.Translate(transform.forward * -1 * Time.deltaTime * speed);
	} else if (moveForward){
		newObject.Translate(transform.forward * 1 * Time.deltaTime * speed);

You would check transform.position for the minimum and maximum values you need. You could have a set of min x,y,z and max x,y,z variables (floats or vector3), or you could use another object, get the ‘bounds’ on it, and check against those. The other object could just be a collider or box. Just set the renderer on it to disabled so it would be invisible. Look at these: