Could I just construct the whole thing out of cardboard?
Answer from Piñata Boy
Yes, you can make the whole thing out of thin corrugated cardboard. If you’re making a piñata similar to the one in the photo above, you will want to fortify the seams where the pieces of cardboard come together. You can do that with masking tape covered in papier-mâché. You can also reinforce the corners with masking tape alone, but the seams where the cardboard comes together are already going to be the weakest points in the piñata, so you will want to make sure you apply enough layers of masking tape to really strengthen those joints. If you feel the cardboard is too tough to break, just punch some holes in it or score it with a razor blade to weaken it before decorating.
You can find large sheets of thin corrugated cardboard for free at Sam’s Club or Costco. The stacks of paper towel and toilet paper in these stores have large sheets of thin cardboard and thin corrugated cardboard inserted between layers in the stacks, and they will let you take this cardboard for free. I would not use ordinary corrugated cardboard for this, however, because it can be very hard to break even for high school students. I would look for the thinnest corrugated cardboard I could find.
If you can reinforce the seams sufficiently with masking tape, you don’t need to use papier-mâché at all, but if the project requires using papier-mâché, you can use some strips to close up the seams.
You can also think about making a rounded banana instead of this flat one. A rounded one will require papier-mâché and balloons or a mole of some kind. This will take more time but will result in a more realistic-looking banana. You might be able to find a banana-shaped Mylar balloon at a party store that can serve as the banana portion, and then add peels using something like thin cardboard or posterboard. The peels would not require any papier-mâché to stiffen them up since they are really just added decorations and are not part of the candy container itself. Their flopping around will add movement to the piñata during the smashing.