2D Tilebased game with blocks. Only allow to place in corners.

Hello everyone! Im trying to make a tilebased game much like the board game Blockus. I know there are many like it but im making it to learn unity better.

I'm trying to make the blocks only get placed so the corners are the only thing the can touch.

Not like this:

I have colliders on tht block itself and a seperate child gameobject with colliders on the corner so that its only allowed to be placed when triggering with the corner collider. But it still gets placed alongside the other block. I know i can just add another collider to prevent this, but its alot of work since there is about 20 different blocks.

Is there a way to script it so it only places in the corners?

Here is some of the placement script:

 public static GameObject selectedTile;
 
     private Vector2 mousePos ;
     private LevelBoard levelBoard;
 
     bool isColliding = false;
     bool tileIsSelected = false;
     bool hasBeenPlaced = false;
 
 
     void Update(){
 
         if(tileIsSelected == true && hasBeenPlaced == false){
             mousePos = Camera.main.ScreenToWorldPoint (Input.mousePosition);
             transform.position = new Vector2 (Mathf.Round (mousePos.x), Mathf.Round (mousePos.y));
         }
 
         if(tileIsSelected == true)
         {
             SelectedTile ();
         }
     }
 
     void OnMouseDown(){
         tileIsSelected = true;
     }
 
     void OnTriggerEnter2D(Collider2D triggerEnter){
         if (triggerEnter.gameObject.tag == "PlayerTile") {
             isColliding = true;
             Debug.Log ("Triggering with" + name)
         }
     }
 
     void OnTriggerStay2D(Collider2D triggerStay){
         if(triggerStay.gameObject.tag == "PlayerTile"){
             isColliding = true;
             if (tileIsSelected == true) {
                 ChangeColorRed ();
             }
         }
 
     }
     void OnTriggerExit2D(Collider2D triggerExit){
 
         if (triggerExit.gameObject.tag == "PlayerTile"){
             isColliding = false;
             if (tileIsSelected == true) {
                 ChangeColorBack ();
             }
         }
     }
 
 
     void SelectedTile(){
         
         if (tileIsSelected == true) {
             
             if (Input.GetMouseButtonDown (1)) {
                 transform.Rotate (Vector3.forward, 90f);
             }
 
             if (Input.GetMouseButtonDown (0) && isColliding == false && LevelBoard.tileOnLevelBoard == true 
                 && LevelBorder.tileOnLevelBorder == false && AllowedColliders.allowedPlacement == true) {
 
                 SpriteRenderer[] renderer = GetComponentsInChildren <SpriteRenderer> ();
                 foreach(SpriteRenderer r in renderer){
                     r.color = new Vector4 (r.color.r, r.color.g, r.color.b, 255);
                 }
 
                 tileIsSelected = false;
                 hasBeenPlaced = true;
                 LevelBoard.tileOnLevelBoard = false;
 
             }
         }
     }

Thanks for your time !