There are small, commercial units for cleaning jewelry, $30-40 range. I use one to clean my electric shaver, and a few other things, $18 surplus years ago. You need to decide how big you need the tank portion, most I've seen are stainless steel, must be some reason for that, beside being shiny and corrosion resistant. Then you need to find the volume of your container, should leave some splash room. Next step, would determining how much power you need from the transducer. Too little, you might as well scrub by hand, too much and you have a messy fog machine. The tough part, sourcing the appropriate transducer, which should come with a spec sheet, from which you can design and build the driver. I've look into both ultrasonic cleaning and fog machines, both very similar. Been a few years, and transducers were a little illusive in my price range. Piezo speakers weren't design for this, and doubt they can handle the load.