Those shells always feel real flimsy to me. They usually don't latch on right and there'll be gaps and creaking. I don't like 'em. Besides, it's not like the 3DS is super expensive gear that needs to be protected from any and all bumps or scratches. I'd say just learn to let go of that stuff man. The plastic can take it. Adds some character too.
Now, protecting the screen I understand. It's annoying when there's scratches and stuff on there, it distracts from whatever is happening on the screen. My 2DS just has a Hori protector on the touch screen to add a bit of protection for when they stylus starts prodding it. I was able to pick up a bunch of 'em for cheap on Play-Asia so I could waste a few if dust ended up under it. That was a while back though.