Unfortunately from the current api it seems you can only draw the first slot as mousedestination returns the first item in the inventory (L->R,T->B)
But in order not to overlap the number, you can do something like:
g.drawRect(
(int) this.inventory
.getMouseDestination(
this.inventory.getSlot(item))
.getBoundingBox().getX() - 1,
(int) this.inventory
.getMouseDestination(
this.inventory.getSlot(item))
.getBoundingBox().getY() - 1,
(int) this.inventory
.getMouseDestination(
this.inventory.getSlot(item))
.getBoundingBox().getWidth() + 2,
(int) this.inventory
.getMouseDestination(
this.inventory.getSlot(item))
.getBoundingBox().getHeight() + 2);
where the additions correct the rectangle dimensions
apa