Are there any openings at the top for lightning? I'm not sure how the 6' cases are put together, but the IKEA one has a simple hole at the top where you can attach a light source. My buddy somehow replaced the bottom ledge with a smoke plexiglass thing and stuck a light underneath to illuminate the bottom, and has rope lights along the top. I'm not sure if all of the 6' cases are the same though.
What about the ceiling of your display? Maybe it's just photo trickery, but it looks like there's a hole at the top. Stick lights would look nice too! That would balance out the amount of light pretty well. You may get a mess of wires though if you do that. Wish I could help more .
Edit: Just saw your other post. Those look cool! I was at a Home Depot last week looking for lights and found a set of 3 for 19.99, wired. What's cool is that if you don't need all three, you can pull them apart I believe.