In that case, I would say just make sure to put a telephone within her reach and the keys to any locks (you can give those to a trusted friend and give her that number, I suppose, but you should be able to trust her to not let herself out at her convenience if she is a good slave) so that if she does have an emergency (which she shouldn't but you never know) she can call out for help and get out. If the garden is secluded enough and had a good spot to do so, I would even suggest going to the bathroom outside. Just a suggestion.