We have lever doorknobs throughout our house, partially for accessibility reasons. While they're easier for me to open, they're also easier for our clever toddler (and apparently cats) to open. I looked around, and commercial systems try to prevent use of the doorknob completely and, if youtube videos are to be believed, they're ineffective at that. I didn't want something that could lock an occupant inside the room. This simple device blocks the lever from being pulled down, while still leaving it free to move up. Our short little toddler isn't big enough to push the lever up, so he's kept out, while us giants are free to pass.

This model is designed to fit a Schlage Flair with a round plate, but it'll likely fit many other models and brands. You may have to scale up a small percent if your doorknob backing plate is a different size.

Critical default dimensions (scale up or down as needed)
Maximum backing plate diameter: 29mm
Distance from door surface to back of lever: 47mm

Installation

• Remove doorknob
• Slide childproof device behind doorknob
• Re-install doorknob, leave screws slightly loose
• Rotate device so that raised portion is below the lever and the lever is just barely in contact with the raised portion
• Tighten all screws
• Test to make sure device doesn't slip. If it slips, tighten screws some more. If you can't get it tight enough for it not to slip, do not rely on lock for safety. Please remix and create some mechanism that prevents the device from slipping.

Alternative Mounting
Check out the remix by arundolon that uses an alternative mounting. For handles that have enough surface area, it allows you to install without removing the doorknob.