Child Custody Arrangements Around Shiftwork - Logistics of Coordinating

I have a loose arrangement ATM regarding custody of my girls with XDP and I want a firmer one.

He can't do many overnight stays because of his roster.

One week he's on early shifts which within the same week could vary from a 4am to a 10 am start.
The next week he's on late shifts which within the same week could vary from an 11am to a 6pm start.

So the only nights where he can have them is on the weekend. However, he often works on Saturdays or Sundays and even on the days where he's nominally free, he may want to have the freedom to take an extra shift for the overtime money. He can often make $400-500 by taking an extra shift.

Long story short. He prefers a very loose arrangement where I do all the running and have to ask him every day what the plans for tomorrow are. He has never initiated contact regarding what time he is having the girls the next day.

When I have been able to pin him down he says he would prefer to share an electronic calendar. We tried one of these when we were together and there were always glitches. He also can't tell me what will be the protocol if one of us wants to change it. So, for example, if the calendar says he has them on Saturday in two weeks time, what's to stop him just going in to change that without consulting with me?

So, personally, as old-fashioned as it is, I would prefer to have a paper version that gets emailed. That way I have proof of what we agreed and changes have to be approved by the other person before another version gets issued.

How do other people handle this. Obviously with very routine custody arrangements, there would not be this problem but we have a situation where the hours that he looks after the girls varies every day and every weekend.