Binder clips are great for sealing bags. In the freezer, you can also use them to hang the bags and create some more space.
Redditor Roxanne712 shows how to hang bags from the shelves of your freezer. The tricky part is keeping the bag sealed while putting it on the rung. You need coordination to clip the bag while in the freezer, but once you get it, you can make your space much more usable. Added bonus: this way the bag won't get stuck in the back of the freezer.