I’m looking for a compact, non-contact level sensor, nothing fancy, just full/empty. Any suggestions?

You should take a look at the contactless ultrasonic level sensors offered by Sonotec. They can be outfitted to your existing vessels, pipes and the like with no concern about compromising integrity or sterility. Also, the performance is better than just full/empty so you can avoid mostly empty and more than full situations. The link below takes you to more information.