I recently had some help designing an exponential horn for a JBL 15" (don't remember the model) which can fit in a 40 cubic foot cabinet in 1/4 space (against floor and a wall). 40 Hz flare. BTW, if I were you I wouldn't be worried about 18" drivers and transient response... That's very much a myth. Some of the best drivers for SQ have been that size or even larger.
Adam