Learn Spanish (Latin America): Siempre pienso en los dueños del baúl. - I always think about the owners of the trunk.

In Spanish, the verb pensar (to think) pairs specifically with the preposition en when you are talking about having someone or something in your thoughts. Verbs often have their own specific prepositions they "govern" or require. So, even though sobre generally means "about," the set phrase for "to think about" is always pensar en.

Could I say pienso de los dueños instead?

Not in this sentence. You use pensar en when someone or something is occupying your mind. You use pensar de when you are asking for or giving an opinion. If you said pienso de los dueños, it would mean "I have an opinion about the owners" rather than "I am directing my thoughts toward them."

Just a refresher, why do we use del here instead of de el?

Whenever the preposition de (of) is followed right away by the masculine singular article el (the), they must combine to form del. This is a strict rule in Spanish to make pronunciation flow better, so de el baúl automatically becomes del baúl.

Why does the word baúl need an accent mark?