Each of these multiple horns should be designed as a single horn with a single driver. Each single horn should have sufficient length to support ¼ wl of the lowest frequency you wish to reproduce.

You will also need to calculate the required mouth area of the large horn to reproduce the lowest frequency desired. Divide this mouth area by the actual mouth area of one of your single horns. This will yield the number of single box horns that would be required to be combined together to obtain the mouth area needed for the large horn.

As mentioned elsewhere each single horns length could be shortened anywhere between .6 and .8 corresponding to the amount of single horns needed to create the large horn. Fewer single horns would dictate less shortening, with larger amounts of single horns needing to be shortened by a greater existent for the same given fo. Alternatively each single horns length could be left intact and you would benefit from the new lower fo.