The Metolius Waste Case is the most convenient option for a big wall turd cage. You get the whole container and the wag bags in one purchase. The bag is bomber, can easily be clipped under the haul bag and, best of all, you don't have to worry about losing the lid as you do with a standard poop tube. For gear junkies it's also kinda cool to have another mini haul bag.
Other than being a little expensive, the main drawback on this device is it is not perfectly air tight. The river bag closure does contain most of the odor but as temps heat up in the Valley on Day 4 of your wall, you will probably get a whiff of yesterday's business. Also, the size of this bag is good for most big wall ascents but if you have a team of three and are going to be on the wall for days, you will want either a bigger bag or two of these.
This bag is ideal for a two-person big wall team that will be on the wall for one to four nights or a three-person team for two to three nights. If you are going to spend more time than that, bring another poop tube or a river bag for backup.
While this is one of the more expensive options, it is a good value if you consider its durability and the time you save not making your own poop tube. Sure you can make your own for $10 in parts. But if you have to make a trip or two to the hardware store and don't get it right the first time, you could end up spending hours making your own. Most importantly, unless you have some supervision from an experienced poop-tube maker, you will be taking a gamble on a making a device that might let you down on the wall…eeooouuwww.