Thanks you so much for the details, Hattiegun.
I plan to do something like yours. I believe Menards sell the 6 mil heavy plastic. I am going to use the polycarbonate panels (from Menards) for the run roof. However, I cannot do a steep roof like yours because my rafters are already in there...
For the stock tank in the pen, I piled sand around it so it is kind of a slope for them to get in and out.
Outside the pen, we have two more ponds (with fish). The ducks go into these most of the time (without hurting any fish).
For the Michigan weather, the Rubbermaid stock tank from TSC or Menards will last pretty much for ever. They sell some in 300 gallon size and I have used in my koi ponds (as filters) for over 10 years and they are still not leaking or breaking.
These come with a side drain near the bottom...