These cheap little boot shapers are great! I ended up buying a second set for additional boots. My only complaint is they can be a little annoying to put into the boot if you have the zipper down, you kind of have to make a cone shape with the plastic and hold it while you zip (which can be quite challenging depending on the zipper).

While I agree with other commenters that a pizza box lid would probably also do the same thing, these are nice and handy. If you have shorter boots, you can use the punch out so that the shaper is the perfect size for your boots.