My cat never had any difficulty to break the door of its cat carrier. Each travel to the vet lead to driving with a cat playing with the brake pedal. To avoid a disaster, I had to design an incredibly strong door. If you happen to own exactly the same cat carrier, that may help. (Illustration : design proof ; a cat in a box watching its favorite meal, tuna fish, outside the box. Cat lovers may be reassured to know that the cat was allowed to eat the tuna fish at the end of the experiment) Use 4 M4x15 bolts. The design includes hex holes to ease screwing.