Lillian (a 10ish pound giant angora) has the bottom of a small cage as her litter pan. She was having issues with the large sized cat one and I had the cage in the garage so just used the bottom. It covers about 1/4 of her cage, so is about 27X14 inches. I would only recommend one if you can get it cheap, I paid about $3 for the cage used.
If you use a storage container, I would try to avoid ones with grooves in the bottom. They can be hard to clean.
If there are garage sales near you, try some of them. You might find something suitable for a litter box. You might want to measure the cage before you go out to make sure it will fit.