I built a makeshift fence for Cooper from materials I bought from Lowes. I bought these
Shop Garden Zone 29in x 38in Empire Fence Base Unit at Lowes.com and used their fitted iron stakes to secure them into the ground.
Cooper was about 7 pounds when I built this and felt secure that he wasn't going to get out. If the spaces seem to wide, you can perhaps try something like this?:
Shop Garden Zone 44in x 71in Decorative Leaf Design Garden Fence Panel at Lowes.com but I'm not sure how that goes into the ground.