I used a lot of broken down boxes that were folded up and packing paper to make a solid inside. Our whole family can sit on it and it holds up just fine! I used a hot glue gun to put it together. Then covered it with scrapbook paper. It needed to be recovered about once a year. It was time consuming and the scrapbook paper was expensive, so I started looking for a more permanent solution.
I bought some cushions, sewed them together to make it fit on the front of the couch. I then had a couch cover that I used a staple gun to secure the cushion to the couch. Then I put an additional couch cover over that and added some pillows.
We had made the pillows years ago, the couch covers I got at a really good clearance deals, and the cushions were the most expensive part of the project. I know it doesn't look like a Ethan Allen couch, but it provides a cozy place to rest and some extra sitting room for our family. Here is how it turned out.