I also use tree branches but you can use a 2x4, either narrow or wide side up. Just round off the corners to make it more comfortable to them and to get rid of splinters. I suggest sand paper for the rounding.
I find it convenient to be able to remove mine easily if I want to. Drill a hole in the roost and the support and put a 5" nail through the hole to keep it in place.
I use a 2x4 mounted on the wall of the coop by shelving brackets. If you live in a place where you get cold winters, I recommend building a roost where your chickens can cover their feet at night. Something like a 2x4 works great. The reason for this is to prevent frost bite in your chickens feet.
I use flat 2x4s. Many folks will say to round them is used flat. I've never done that and never had a problem. One thing I do when buying 2x4s is look for some that are naturally rounded because of bark removal, etc.
I have tree branches, 2x4's wide side and narrow side up. It seems that more and more of my chickens are switching to the branches. If your hen has no toes I would say give a variety and see what she chooses.
The top roost in pic below is a 2x4 wide side up and the bottom roost is a 2x2