does the resevoir for the rear shock have to be mounted horizontally or can it be mounting vertically, with the hose on the bottom?

The rear fender that it normally mounts to was gone when I got the bike, its zip tied under the seat. However, I have an undertray that wont fit with that reseevoir in there. I was thinking of mounting it to the right pass peg bracket, but may have to do it vertically so it stays.

 

It pressurised gas, so I think you can do it.