here you can learn how to make spherical wooden horns, explained step by step ( thanks for John Haskin for his precious informations ) :http://www.flickr.com/photos/7808268@N07/
if you decide to make a pair, i can email you corel graph's for tractrix horns with 88cm diameter ( 150hz ) and 58cm ( 250hz ) horns.
John Hasquin does not recommend to make them bigger than with 88cm diameter :
" I would advise you not to make a horn any bigger than 88cm mouth. The 95cm mouth horn is too big and you will not be able to get good high frequency out of it. A 95cm mouth horn with 10cm throat will be about 92cm deep. For a horn that is between 80cm to 73cm deep, you will need a very efficient speaker like the Fane Studio 8M or the Ciare 8.5 Nd MR " .
If you do not have a lathe, you can use a file, and make them manually, as explained at vincent.brient site: http://vincent.brient.free.fr/pav_ronds.htm