Positioning Prefabs in exact positions

HI all, Im at my wits end here. Im sure its something simple that Im missing. Any help would be SUPER appreciated.

So I have a prefab,its called wall.It consists of an empty parent object, a super basic 3d mesh(imported from max) which is a wall section and a box collider that covers the wall mesh. What I want is to instantiate 4 of these in a square formation. The idea being to generate a few of these to build a tunnel.The problem is positioning the prefabs.They just dont fit together. The prefabs overlap. One odd thing, the scale of the mesh in the inspector is 0.0254(x scale for example),but the x scale of the collider is something like 59. Also the z offset of the collider centre is non zero for some reason,if I set it to zero then it doesnt cover the mesh . Here is the code I use to instantiate:

    float wallHeight = wall.transform.Find("Wall Side").GetComponent<BoxCollider>().size.y;
             float wallLength = wall.transform.Find("Wall Side").GetComponent<BoxCollider>().size.x * 
                 wall.transform.Find("Wall Side").localScale.x;
 
             Debug.Log("loopSidelength : " + wallLength);
     
             Vector3 p = new Vector3(0,0,0);
             
             //RHS
             p.x = centrePoint.x + wallLength/2f;
             p.y = centrePoint.y;
             p.z = centrePoint.z;
             GameObject right = (Instantiate(wall,p,Quaternion.Euler (0,270,0)) as GameObject);
             right.name ="Right";
     
             //LHS
             p.x = centrePoint.x - wallLength/2f;
             p.y = centrePoint.y;
             p.z = centrePoint.z;
             GameObject left = (Instantiate(wall,p,Quaternion.Euler (0,90,0)) as GameObject);
             left.name = "Left";
     
             //Top
             p.x = centrePoint.x;
             p.y = centrePoint.y;
             p.z = centrePoint.z + wallLength/2f;
             GameObject top = (Instantiate(wall,p,Quaternion.Euler (0,180,0)) as GameObject);
             top.name = "Top";
     
             //Bottom
             p.x = centrePoint.x;
             p.y = centrePoint.y;
             p.z = centrePoint.z - wallLength/2f
                 ;
             GameObject bottom = (Instantiate(wall,p,Quaternion.Euler (0,0,0)) as GameObject);
             bottom.name ="Bottom";