Hi Joe et all,
having built and listened to the Zigmahornet and the Swan 101a (FE108EZ) and also the Fostex recommended backhorn for FE103, my opinion is as follows.
The Zigma is a very nice speaker, yes, in comparison to the 101a it lacks bass but on its own its not that bad. If you need bass then add a sub.
The 101a (which my wife refers to as E.T.) is so far my overall favorite of all the 4" based enclosures I have built. Clean and clear with great bass, yes bass, looking at the driver you would not believe it but run some test tones through it and listen. It does however take some effort as there are IIRC some 30+ pieces of wood per cab but I think its well worth the effort.
The Zigma is much easier.
The Fostex recommended horn does not work for me, simply put it does not sound right and nothing I did ever did made it sound right.
They have been to returned to the soil from whence they came.
I still think the FE103e is the better of the two drivers though, its midrange truly shines.
Maybe with more time the 108EZ would sound as good in the mids but I somehow hold little hope in that regard.
BTW, 15mm plywood is sold as 5/8th's. Almost all plywood is metric nowadays and much of it from Brazil. 13mm is sold as 1/2" etc.
Andrew