Denali split soundbomb works well, you will have to fabricate a couple of brackets.
This is a 2013 bike and I have copied this from a post of mine on another forum.
Having already upgraded from the stock single horn on the FJR to the Piaa twin sport horns.
https://www.fjrcatalogue.com/catalog/item/7379389/9589221.htm
I felt as though I wanted a bit more oomph, I had installed the Fiamm Ultimate Blast on my BMW K1200GT and liked it so wanted something similar.
So I got myself the Denali Split Soundbomb.
https://rg-racing.com/browsebike/Yamaha/FJR1300/2013/DENTT-SB10100B/
This is not an install for the feint-hearted, the parts need to go in exactly the right place as the space is tight. I made a couple of brackets from aluminium and here are the photos.
Compressor on a custom bracket
Close up of compressor and bracket and you can see the hose routing to the horn
Better view of the hose route
Horn and Bracket
Relay under right side panel
Finished job with fairings back on
I did a sound comparison test and the PIAAs were putting out 106 Db compared to the Split Soundbomb which gives 119.7 Db so I'm very happy with that.