in my spotteds pen, the mixing tub is indeed a cement mixxing tub. I in fact use these in all four of my larger pens. My younger turtles have the sloped paint roller tray, a green one called a "big ben" roller tray, it's extra wide, and I filled the trench at it's entrance with gravel so they would not fall in there.
The cement mixing tubs are very easy to clean. I go out every weekend, remove the cement block step and scoop out the water with a watering can (as much as I can) and water my willow tree or whatever I want with the gunky water. Then I pull out the tub, spray it out with a hose (every few weeks I will scrub it with a brush) and put it back in, replace the cement block, and fill with water.
I have seen a superior method which I will employ in my newly expanded pens as soon as I finish them. In this method you buy a sink drain flang, silicon, and a rubber sink stopper and install it into the bottom of the tub, so you can simply pull the plug and rinse it out and it will drain on it's own. In my clay soil I will build a wooden frame twice as deep as the tub, but sized so the tub sits on it with it's lip, and fill the lower half with gravel so the water can drain from the tub to the gravel when I pull the plug.
The tub for the spotteds is a very large tub, probably four feet by two feet or so, and my pair does fine in it for now (they are getting a pond soon) and is sharing it now with the two juvenile wood turtles which are the same size as them.