Andrew, how advanced are your carpentry skills? How large is too large? I've built many large horns over the years, and Art is correct, you do need really big horns to reproduce lower freqs efficiently. If you are prepared to transport cabinets as big as 48W x 40T x 72D, then you can seriously consider large horns, and the results will be very, very impressive.
I would suggest you will need to look beyond the small horns you currently have, and consider building mid-bass horns to go with the subs. You can build quite a system using all Eminince/QSC components, and it would be very cost-effective.
Feel free to contact me if I can be of help, as this seems like the kind of project I enjoy.