Hey guys thanks for the all the comments,
I was definately going for a no frills approach, just a simple horn designed to get used. I see all these fancy horns in museums and collections, all thats means is the simple ones got used til they broke.
The Koa wood is pretty hard but it's not that tough to work with, I am currenty making a musket stock from Koa with nothing but hand tools.
The Waiwi or strawberry guava is a bit tougher it tends to gum up any saw blades you use.
As for the horn hanging tip down, I did that because i orignally planned to make the horn removable from the band with the staple in it, so it would just be wedged into the band and sort of held by gravity, while still being removable, but after wearing the horn for awhile and having it come loose i decided to epoxy the band on.