I havnt looked at the 03 but I think its probably the same light that I have on my 01. The hole should be about 1 inch tall and about 3 inches long right?.. you can cap it with fibreglass or some plastic bondo by putting a backing in the hole then filling and sanding it down but it will be apparent that you did this so if you are going for looks I wouldnt. You could also do what I did to fill in the holes that were left when I removed my rear seat belts, find a fender light (the little reflectors on most cars fenders) that fits in the hole and just stick it in

It isnt functional but it fills the hole without leaving an apparent "scar" on the car. you could even wire it to your other trunk light so it lights up when you open the trunk giving you some extra light at night