We just adopted a special needs Flemish Giant. I want to build an indoor pen and this looks like a great option. Are these C&C cage sections? They look bigger than what I am used to seeing.
They are just the grids from wire cube shelving. The photo you posted shows them using the grid connectors that come with the shelving. I advise strongly against that since the connectors are known to pop loose. Use zip ties to connect the grids instead.
You can create basically the same thing with an exercise pen as well.