I've just replaced my doors with brand new nice oak ones. I want to take my time to pick a nice lock, and fit it well. I also had some friends staying, who wanted a bathroom lock.
The conundrum: How do I lock the door without marking it in any way?
The solution: String and tube!
Step 1: How It Works
If the handle can't turn, the latch can't open, and neither can the door.
The first thing I tried, was cutting a length of wood the right length to jam under the handle on the inside. It worked fine, but having a length of wood in the bathroom was confusing to people.
I happened to have some 1" perspex tubing from the RGB fishtank project it fitted nicely oven the handle. Now I just had to tie string onto it, stop it sliding up and down with duct tape, take the string over the door, wrap it around the base of the handle on the other side, then back over the door, tie it onto the tube keeping the string nice and tight, and secure in place with duct tape.
Step 2: The Results
It works great! And it only took a minute or two to make!
It's a bit tight to get onto the handle. But it needs to be, to stop it from moving.
In an emergency you can just cut the string to get in! No damage to the door.
The plastic tube taps against the door as you open and close it. The sharp edges on the tube can slightly mark the varnish. You have to look really closely to see. I'd recommend putting a radius on all the sharp edges to stop this from happening to you.