While fooling around with some 18" pro drivers I found that if you wanted them to play down to 30Hz (F3, not F10), the cab required would be so big that it could also fit a 21. Since most 21" drivers cost just a tad (maybe 10%) more than their 18" equivalent, going with 18" doesn't make much sense imo.
If the cab is too deep for you, you could trade some depth for height. That would extend the horn section, but you might run into issues with the port length.
Maybe make it very tall and compensate for the shorter port with added back chamber volume. If it's tall enough, the horn section could also be a single segment, or well, a big segment and the small segment which opens to the front.