A tapped horn or tapped pipe might be the most efficient way to get 19 Hz, but be prepared for a big box.
Look for a driver with Fs around 27 Hz and Q around 0.4~0.5 and reasonable efficiency. Then use HornResp to model the driver in a tapped horn or tapped pipe. Then, decide on the folding technique that results in a box that will fit where you'd like it to fit :-). For a folding idea, see link above.
