I designed this to keep my young kids from easily getting into rooms we don't want them wandering into (furnace room, laundry room, etc). I wanted something temporary that didn't require damaging the door or walls.

To install, remove the door handle and place behind the plate of the handle on the inside of the room you want to keep the kids from entering. When installed, the door can be opened by pulling the lever up, but not down.

It prints without supports. Initially I wasn't sure if the layer orientation would make it too weak, but my kids have hung from the door levers with their full body weight and it held up. I initially relied only on friction between the door and plate, but once the kids got stronger/heavier I added tabs that line up with a protrusion on the inside of the plate. I don't know how common/standard this is, so you'll want to check your door handle first before using that version.

If your handle plate is a different diameter you can probably get away with just scaling the model for printing.

Also, it should go without saying, but don't trust your kids' lives to this. A smart or lucky kid could still get past it, or it might break. If it's imperative that kids can't get into a room then install a proper lock.