I have the lockblock and it’s about as universal mount as you can get but I never found it terribly stable on the bars, but is about the best thing for the helmet.. then supported with maybe a ziptie for added stability.
Just got one of the gun mounts Nickjb has linked to, as a bar only mount, and it looks pretty good for the money.. though have actually yet to try it out. Available from UK sellers, for quick delivery, but this guy was cheap as chips and delivered in 11 days.. pretty damned impressive for the cash
(I’m sure you could afford one for each of your bikes!)