u/Dry_Prompt_7729

Recently moved to a new apartment with my 2 cats, I now have a balcony I'd like to let my cats to but there is a perfectly cat sized gap where the balcony connects to my neighbor's side. Any suggestions on how to block said gap so my cats don't go on unwarranted adventures to visit my neighbor? I ruled out cardboard and plastic because those will never hold a cat out :D.

For anyone interested the gap is 85cm tall, 14cm wide and 8cm deep