But occasionally left open a little long and other cat gets in.
Stop doing that, then.
Could probably do the RFID cat flap, but a lot of work/expense. Very thick walls = core bit hire, double glazed windows, uPVC doors with all kinds of decoration on
Broadly the same here. I cut through a plain uPVC panel on the kitchen door and fitted a cat flap in minutes (after about a monthās worth of prior angst that I was going to bugger up the door).
Prior to the relatively short-lived Charlie The Rescue Lunatic for whom I installed the flap, I had a cat who lived to the ripe old age of 18 (and destroyed me when she couldnāt any more, the furry little bastard). We, and later I, managed her movements for nearly two decades by opening the door when she miaowed.
If all this šš is nonviable, could she perhaps turn them into house-cats and not let them out? Easier with younger cats though, theyāre generally creatures of habit.