Midwest metal panels. (16) 2'x4' panels (7) 2'x2' panels and (1) 2'x3' panel. 24 panels total. $5 each or $100 for all.
Some of these are connected with metal clips. You can add or remove clips to change the layout. See the photos for replacement clips. They can also be connected by snaps or zip ties.
Most of the panels are connected so that they are 2' high and 4' long; some are loose or connected to the 2' panels to use as gates.