GUI focus control - inventory system issue

Hello everyone, I'm trying to build a game with only keyboard controllers so I wanna create an inventory where you can use arrow keys to "jump" from a box to another. After I created the base for my inventory, it looks like this (without appropriated graphic, ofc ahaha)

The problem is I don't have any idea how to assign a decent GUI.SetNextControlName, because (for what I understood by debug) I can't use it in a for loop.

So, I have these

 private Rect invRect;
 private string[] invBoxes;
 private int boxIndex;

That's the OnGUI part

 void OnGUI(){
         GUI.skin=skin;
 
         if(showInventory){
             invRect=GUI.Window(0,invRect,DrawInventory,"");
             GUI.FocusControl(invBoxes[boxIndex]);
             if(Input.GetKey(KeyCode.Escape))
                showInventory=false;
             InventoryInput();
             Debug.Log (GUI.GetNameOfFocusedControl());
         }
     }

And, finally, this is the DrawInventory method

 void DrawInventory(int windowID){
 
 
         //[...]things not related to the problem
 
         int i=0;
         int BoxIndex=0;
 
         GUIStyle invBox="BaseBox";
         invBox.fixedHeight=invRect.height/18;
         invBox.fixedWidth=invRect.width/2.05f;
         for(int x=0;x<slotsX;x++){
             GUILayout.BeginHorizontal();
 
 
             for(int y=0;y<slotsY;y++){
                 GUILayout.BeginVertical();
                 GUILayout.Box(inventory[i].ItemIcon, invBox);
                 GUI.SetNextControlName("Box"+(BoxIndex));
                 GUI.Box(GUILayoutUtility.GetLastRect(), inventory[i].ItemIcon, invBox);
                 invBoxes.SetValue("Box"+BoxIndex,BoxIndex);
 
                 GUIStyle textGUI="ObjectName";
                 textGUI.fontSize=(int)(invRect.height/22);
                 GUI.Box(GUILayoutUtility.GetLastRect(), "   "+inventory[i].Name,"ObjectName");
 
                 if(inventory[i].IsStackable&&inventory[i].ItemCounter>1){
                     GUIStyle quantityGUI="ItemCounter";
                     quantityGUI.padding.right=(int)(invRect.width/50);
                     quantityGUI.fontSize=(int)(invRect.height/22);
                     GUI.Box(GUILayoutUtility.GetLastRect(), "x"+inventory[i].ItemCounter.ToString(),"ItemCounter");}
                 BoxIndex++;
                 GUILayout.EndVertical();
                 i++;
 
             }
             GUILayout.EndHorizontal();
 
         }
         GUIStyle DescrBox="DescriptionBox";
         DescrBox.fontSize=(int)(invRect.height/22);
         GUILayout.Box("-Select an item-","DescriptionBox");
     }

I've started to develop in unity 2 weeks ago, so if the code is messy, pardon ahaha I've also used a couple of tutorials to understand better some aspects. Btw, I've created a GUILayout.Box and after that a GUI.Box because (I think) that the GUI.SetNextecc. method doesn't consider the first option, so I just used it for the coords.. And all seems to work, BoxIndex increments itself properly, but

 GUI.SetNextControlName("Box"+(BoxIndex));

always remains "Box0". I've tried to edit my boxIndex (with the lowerCase) but it returns an empty string and not "Box1", so it's pointing to nothing.

Am I getting wrong something? Does another method exist to achieve this issue?

Thank you :)