For just about all of 2018 and halfway through this year no one was asked to stay late and no one was called in to cover call offs. Many times I walked into a shit storm at the fitting room because of call outs the shift before mine. Many times I went shopping and saw a shit storm in other areas, talked to my coworkers, same answer. People asked to stay to finish, always a no. Some weeks shifts on the swap shift board would be crossed off; people got that shift off but the store would do without rather than let someone pick it up.
So no, there aren't ways of getting 40.
And if you need people for certain times, schedule them. If your plan is to hold them outside their schedule, don't blame them for saying no. People don't stay in their house, showered and ready to go, doing nothing but vegging in front of the TV waiting for your call. People have many things going on besides work, whether it be another job, a day trip with the family, house work and yardwork that must be done, a chance for that bike ride, coordination for a MMORPG event, or simply giving in to feeling sick and resting after putting their all into work the previous day. If you need them, schedule them in advance. If you have the hours to have them stay, you had the hours in the first place.