Horn speakers are afaik among the most efficient, and they have designs compactly enclosed in boxes, essentially folded horns: https://en.wikipedia.org/wiki/Hor

Horn speakers are afaik among the most efficient, and they have designs compactly enclosed in boxes, essentially folded horns: https://en.wikipedia.org/wiki/Horn_loudspeaker#Exponential https://en.wikipedia.org/wiki/Horn_loudspeaker#Exponential