You can simply go to Style → Bar Style, set Image Size for “Vertical Top Slot Image” and “Vertical Bottom Slot Image” to 0 in “X” for both. Then you can set the thickness to any small value.
Alternatively, you can set a material for the Thumbs image. Inside the material, you can use opacity + UV to reduce the visible part (e.g. hide parts of the material where UV.x < 0.2 or UV.x > 0.8).