I have twins and we find the back a little small for our huge Evenflo car seats. That being said, we use an old (but perfect condition) acrylic blanket folded about 4 times to put under the seats and go across the entire back seat. Never had a problem, but I'm always having to vacuum up raisins, animal crackers, and fruit snacks (gummies).
I recommend getting some good tight sealing sippie cups for later, and enjoy having the back of the seats kicked once you turn the seat front facing.
Also, remember the latch hooks are little cut out pockets on the lower part of the back of the seat. My dealer didn't know where they were.