Thanks for the explanation Acorn. It's probably that as a Xara user of many years (as yourself), long before rounded corners and smart shapes when we had to create them on our own. So this is my method of choice.

I admit I've never dabbled with, nor understood opacity masks and having followed your instructions to the letter did get the exact same result as yourself. I just couldn't understand what I could do with it beyond that.

I always keep a copy of my constructions on the pasteboard should I require further adjustments.

I enjoy looking at your inventive alternatives, thanks.