How to best zest a citrus fruit that was already halved and squeezed?
I find that the classical zester is a rather clumsy tool and needs quite a bit of pressure to use, which is hard to do against a squeezed lemon. 

I zest with a microplane, and I find that it does a fairly good job with squeezed lemons too, even though it's not as comfortable as with a whole lemon. It is sharp enough to peel off the zest pieces after the lemon was squished. 

This may go against your "aspire strips of zest" desire, because the microplane produces finer zest than the classical zester - what you get are still strips, but thinner and shorter. They are actually better suited for baking, but if you want a specific shape and size for decoration or for candying, this might be a problem. Still, it works good for many applications, and I also prefer it on nonsqueezed lemons.