Looking for a system that'd fit in a Jack's Pack Cat (i.e. small), I bought a Scat Packer: a heavy-duty plastic bucket with a threaded lid (with pressure-relief valve), a cheesy little toilet seat with bungy cords, and a clean-out lid with an RV hose attached. It worked fine but seemed like a gouge for the materials that went into it, so I squinted at the components and saw that they were made by M & M Industries (ha!) of Chattanooga.
When I needed more capacity, I ordered a bunch of extra pails (1- 2.5 gal the original size
, 2- 3.5 gal., and 1- 5 gal., along with lids for all and extra gaskets, for $19.61
M&M Industries Life Latch Plastic Pails
The buckets are lightweight and the threaded, gasket-sealed lids are bombproof— no leaks. My favorite setup is to line the toilet stool with a Wagbag (held with a big rubber band) that can be sealed with a twist-tie for transport. When the bag gets about half full, it gets transferred to a larger bucket (with sealed lid and pressure relief valve that I made from a spring-steel bike pantsclip with plastic tubing slipped on, and pop-riveted in place).
Any plastic potty seat that will fit a regular plastic bucket will work with these heavy-duty models (seats available at Cabela's and big-box sporting goods stores).
To rig the pails, at an ag supply I found a powder-coated frame designed to hang a water or feed pail on a fence, that secures the bucket(s) at the rear of my cat frame. Overall, it was damn cheap and it works.
If you don't feel like messing about, the Eco-Safe system is no doubt a good choice.