You don't say what you are trying to separate.

Generally speaking for larger animals cattle panels or goat panels should suffice. Make sure whatever it is can't squeeze between a wall & the panel and get stuck (horns). Also decide which size holes would be best so they don't get their head stuck.

Depending on what it is, I might make a smaller pen with 4 sides inside the stall rather than nailing/screwing/stapling the temporary separator to the walls.

The best part about the panels is they are removeable & reusable anywhere the pen will fit (including the back of a pick up truck).
Brendasue

I have various animals, with various needs, and so meant this as a general question. Currently, I'm separating a mother duck and some ducklings from the rest of the flock. I also have alpacas.

How do you hold your panels in place in the stall?

--Jonathan

And another point about the current situation, there are also wandering chickens about, so the dividers have to be higher than usual. I've got it rigged up now with plywood to 18 inches to hold in the ducklings and then 2x4 welded fencing up to 5 feet to discourage chickens flying in.

--Jonathan