Phew! priceof castings seems to rocket.
Could you fabricate from ,say, 1/2" x 1/2" x 1/8" steel angle and silver solder in any necessary triangular webs. A bit fiddly, but cheap.
Alternatively, horns for other designs seem a bit cheaper, eg those for my 5" Priness of Wales are "only" about £65.
Any scope there?