Thanks @mattvaru. I can see that working nicely in Codepen, but it doesn’t work when adding in the <head> unfortunately. Note, this is even when publishing (as I know that some custom code isn’t visible in the preview). The volume button is the ‘nice to have’. Really it’s the download I’m more keen for.
I guess that I can either persevere with trying to get the download to work within Plyr, or alternatively I can just add another button (outside Plyr) as a normal element in webflow (similar to as suggested here.