http://www.ustoy.com/coffin-slime-with-spiders
Seriously, they're super fun and great for zombies.
After Dinobot educated me on the difference, I had to go back and see in which the Bride was buried. And it's really hard to tell, there are no clear external shots of the box.
But in Dinobot's picture above, look just to the left of the flashlight beam: that's looks like a corner. There is also a hint of shape in this pic:
Besides, a coffin is more fitting considering the wild west theme of her segment with Bud.
I always thought caskets, or as PL says "A good ol' pine box" was more western, and a coffin was more European?
Regardless, I went back and watched it after you mentioned it, and I'm pretty sure you're right. Check out this scene from the 0:17 mark. It is almost certainly a coffin, not a casket. There's a seam that wouldn't be there otherwise.
EDIT: And after seeing this scene again, let me just say once more how incredibly excited I am for Kill Bill Minimates. This is truly one of my favorite movies.