When I set a broody, I fasten her on her clutch (I make a nest in an old drawer) in a dim (not completely dark) cupboard with food and water. Once a day, whilst I am doing chores and can supervise her, I open it to let her out. The increased light when the door is opened stimulates her to get off and go out with the flock and do whatever is necessary. I close the cupboard door again to prevent other hens laying in her nest and/or messing with her food and water. When she starts to head back into the hen house, I go in and open the cupboard door for her to climb back onto the nest. If by any chance (as sometimes happens during the first couple of days) she goes into another nest box instead of her nest in the cupboard, I wait until she settles and then lift her out and put her on her proper nest. The cupboard door has a few large holes drilled in it to let a bit of light and air in but keeps her protected from more dominant hens.... my broodies are mostly lower down the pecking order.
This system works really well for me. I can see that she gets off every day to eat and dust bath and relieve herself and I know that she is safely back on her eggs when I leave with no risk of her clutch being abandoned or added to or broken by other hens climbing on to them. It's usually just a matter of 15-20 mins and if I let her out when I feed the other hens, she gets to forage with them so they don't forget her and there are no re integration problems. By the time I have finished my chores she is back on the nest and I can leave knowing that nothing untoward can happen whilst I'm not there. There is room in the cupboard around the perimeter of the nest for food and water as I said and to poop if necessary but it doesn't generally happen. After a few days they get into a routine of coming off within a minute or so of the door opening and it makes me laugh to watch them going back to the nest because they look into the other nest boxes and then in the cupboard and then look about to see if anyone is watching before they hop back on and settle down.....you've got to love broody hens!!