No reason you can't use a night light. It's preferable that it not shine brightly on their cage, but when I think of a night-light, what most people mean is not
what you'd call "bright"). Actually, some of my snakes enjoy their own "night-lights" (either black or red incandescent bulbs that are dimmed down a lot just
to provide some overhead warmth & a little light goes along with it) because they light to sleep in their branches & platform-baskets. (not BPs, mine are rat
snakes & a spotted python, etc.- but no real difference)
You could, as you said, put a towel over your snake's cage (or prop a poster board against the side) but unless it's close to the cage & fairly bright, your
snake is not likely to care. Outside there is also a full moon now & then.