I use 17L hard sided stackable HDPE cubes.
These are the empties from Fresh Wort Kits (which are basically commercial no-chill cubes)
Such as this one
I also have one Willow 23L Water Jerry
Which is good for a single undiluted batch, but I prefer the utility of the stackable cubes.
The cubes essentially cost me a 2$ forfeited deposit each, and I slighty concentrate my wort so one 17L cube makes 20L of wort, which fills a keg perfectly.
Others use 20L cubes which are slightly more cubic slightly wide/deeper and shorter
http://www.craftbrewer.com.au/shop/details.asp?PID=2939" onclick="window.open(this.href);return false;
CraftBrewer also supplies 20L wort kits which for 49$ come in the same cube they sell for 17.90$