Mask image

The mask image can be used to cover the design that is outside the product

To create a mask image, simply make the design area transparent like this

That way, the excess design will be covered by the mask opaque area like so

So anything that goes outside the design area will be covered by the white area of the mask

You can include some semi transparent shadows in the mask to give a more realistic look to the final result but that would require some graphics editing knowledge