I tried cutting the shape out with a Dremel tool. Smelly, slow and tricky. A normal wood coping saw ended up working best.
The horn was 3/16" thick, so not too much thicker than it needed to be. I used an inverted reciprocal sander with 60 grit sandpaper to shave it down to about 1/8", but I had cut the pieces out first. Probably would have been beter to do that first, then cut the pieces out.
Then sanding ( a lot by hand, some with the above rec. sander ) to take the angular cutout shapes and smooth them into the desired shape.
There are probably better ways to have done all this.