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You and your cat will both benefit from having the cat flap inside your bifold door. It can also prevent extreme heat or cold from entering your home. First, you must determine the measurements of your pet to determine the right size door flap you need. This will enable you to create a hole that is adequate for the flap. Outdoor Access While installing a cat flap on an external back door is not a job for a novice, those with little DIY experience can usually manage it. This will save you the expense and hassle of calling in an expert joiner,