If you don't already have an armrest,
I have found this one to be excellent.
Sturdy and larger flip-top storage and
comfortable for two arms or elbows.
It was an easy :15 minute install with a T-40 Torx
(and I replaced the matching leather cover
with WetOkole seatcover material over Memory Foam). HERE
is a MotoringFile Review of it.
I bought an R56 version of it last year and love it.
Two sources I am aware of are Minspeed
I have also seen them on eBay,
but you could likely find a UK or EU site to buy from.