Can I ask why you'd do this?
@MrsNuckyThompson to stop them getting onto a road and run over, to stop them getting into other peoples gardens and houses and being a bother to other people or harm coming to the cat, to protect wildlife, to stop people stealing them, to stop the cat poisoner, to make them better 'pets' since they will be in the garden rather than in Mrs Smith at no40's garden!
People keep cats as flat cats, having an enclosed garden isn't much of a deprivation of liberty especially in an urban environment.