The Willow 20L blue ones at Big W actually hold more like 24L. so may be more suitable for the OP. I used to use them all the time as cubes, but when I went to exclusively kegging they were too big so I disposed of them.
ed: they are very robust, and for no chilling they are the best shape, being jerry shaped rather than square they have a far better surface to volume ratio for radiating heat. Never had any problem pitching next day after putting them out the back door at night.