[speedwell68]

I know this is an old post, but it deserves an answer.  I recently got the Official OSMC remote and it works exactly as the OP wants.  I have tried it on my LibreELEC based Raspberry PIs, my Linux desktop and my Daughter's Windows 10 desktop and it works a treat and only controls Kodi's volume and not that of the system.  It is a super slimline and simple remote too.

https://osmc.tv/store/product/osmc-remote-control/