A cave. Driftwood. Plecos like to hide out during the day, which is why caves are such a nice thing for them to have. It's best to have a few of them available so that they can easily hide whenever they need to, which will help make them feel secure and more likely to come out more often. Driftwood is also very nice since it provides a place to hide if it is large enough and stacked/leaned together. Also, many plecos require wood in their diet (can't remember right now if this is so for tigers, but can be easily looked up on Planet Catfish), but since it's not always known what exactly they eat, it's best to provide at least some just in case.