Umm i know a lot of people use the rubbermade garbage cans because you can buy the wheels for them. Toss an extra pump in the bottom with a short (6' or so) clear tube on the end. Let the pump do the mixing and then when you are ready to do PWC
you can wheel the bin out and use the pump to add the new water.
If that is to much for you the same concept will work with any other rubbermade bin. Just make sure you give the water at least 10 hours or so to mix so it stabilizes. Adding a heater to try and get the temp close to the tank is also a good idea.