The last four custom crops remain in the aspect ratio memory until they are crowded out by new arrivals.
Crops cannot be included in presets, but they can be copied or synced from one file to others. You could apply the crop to an image and put it into a collection called, say, "Custom Crop". Then whenever you want to use that crop drag photos into the collection, select all of them with the cropped file as most selected, and sync the crop.