Here is a cool option for mixing concrete that I stumbled upon at the World of Concrete show. It’s called the Maddog from RHM Ventures LLC: a portable handheld mixer that’s designed to work with this specific trough (which also has a lid). It’s available in gas or electric model (for indoors and runs slower so you don’t kick up a bunch of dust inside). According to the manufacturer, you can mix 120 to 180 pounds of concrete at a time. The gas unit will mix 120 pounds in about 2 to 3 minutes. The whole kit weighs about 40 pounds and costs $1,195 (though it has been on sale for $975 at times).
Manufacturer's specs:
Engine Brand: Honda GX25 4-cycle (gas)
Fuel Type: Gasoline or Electric
Weight: 39 lb. Total weight of complete system (mixer, mixing tub, and tub lid)
Trough Length: 43.4375 inches
Trough width: 19 inches
Trough height: 13.5 inches