For the last 10 years we have not had a cat flap and we have four cats. I have an emergency litter tray which they tend to use when its too wet for them to want to go out, I have even known them run in for a pee. They have their routine, Tom goes out in morning, come back at lunch and stays till just after tea then come in a 10pm. We have a diva who just pops out and come straight back in and spends school home time patrolling the pavement for people to tell her how beautiful she is whilst she rolls in front of them. We have moved in to temporary accommodation so one had decided no way is she going outside in case we move again while she's out.
I use washing up bowls as litter trays, they are cheap and the litter seems not to end up on the floor as much. When we move again we will have eight, that should be fun.