I've seen that done a numbe of times, but I never really learned the "how" of it. A neighbor did it (or at least something very similar) too with a stud horse to cut him. Looks pretty simple.
About the same way I do... I use the sidewall cutout from my tire tank waterers, and throw a tub from a protien tub into it, and dump in the salt/mineral. No rain protection... but don't feel it's necessary, even here in our Minnesota weather. That sidewall circle helps to keep the tub in place.