Horn:
I used a piece of steel bar stock, maybe 2" to 3" long, 3/4" wide, and maybe 1/8" thick (these are approximate dimensions). A hole drilled in each end, painted black.
There was an existing hole in the frame where the horn is mounted, that was drilled slightly larger and tapped. The photo is a little blurry due to being enlarged.


Alternator Cover:
The OEM cover is still available from various vendors for around $40. Part No. 21333-MZ0-A00. For example, Ron Ayers lists it for $35.98 plus $5 shipping. I would have quoted HDL, but they seem to be experiencing web problems. I am not aware of any non-OEM replacement part.