I suggest waterslide decals, and put them on the front side not the top. You can get waterslide paper that works in inkjet/laser printer and can make the labels in any font or color your printer can do. They can look as nice as a manufactured product, and they can last a pretty long time as side print even without a clear coat, and are easy enough to remove when you don't want them there anymore.
If you have white caps, you could use dry transfers instead. You can find them in many different font styles and sizes. They are also easy to apply and remove. As with waterslides they are not super durable so side placement is also advised there.
