Looking at the 2nd pic, I think the design would benefit with a couple of diagonal braces.
What if the roof was raised up for better clearance?
How about a gutter at the low end of the roof to collect rainwater, leading it into a
water tub. It might save a little effort in watering the beasts.
a couple of
pallets, a couple of studs, some roof covering, its pretty simple.
I'm thinking that if a drum is not available, a plywood box would do the job.
I would also add a paint job. Give it an extra few years of life.