plena

What fills the thing takes de or je; both are correct, and de is far commoner. Plena also means complete, which is why the great dictionary of the language is the Plena Ilustrita Vortaro and a collected works is a plena verkaro. The root gives plenigi (to fill), malplena (empty), pleneco (fullness) and plenumi, to fulfil or carry out - a promise, a duty, a wish. That last one is a separate word worth learning on its own; you cannot guess it from "full".