Sorry, I had to leave last night to make dinner.

The 2X4's for your pen sound good. I wasn't taking into consideration how big your posts are.
@Ridgerunner - the 1x3's work just fine. They are nailed together to form a frame, the wire is then stapled to the frame to form a panel. The panels are then nailed together. There are supports going across that hold up a roof panel, nailed to the wall panels. We have a odd shaped coop/run due to our yard being sloped. There is a section that is hinged due to an extension piece having to be at an odd angle. At some point I will post photos