Few months ago I bought a bicycle bag from ebay to have my tools everywhere I go. Ofcourse with time I kept adding some tools that worth some money and I was starting to feel that somebody someday will put his hand inside and steal everything. So I had an epiphany one day and put a plastic bag inside. This plastic bag can be used as a saddle watercoat in rainy days and it hides great the tools inside so the potentioal thief, when he opens it, will find just a muddy bag (it’s always handy ). Cool, isn’t it? So far, no theft!
Alternative version: Hugo down in the comments suggested a sock instead of the plastic bag. It’s a good idea because it offers a lot of uses and it’s a wonderful detterent because noone would want to touch it.