I am actually converting from Xflow to Zetec and need to rebuild my fuel supply. I have an old oil catch tank lying around here, which I could use as a Swirl Pot.
It is only 0.5l and I am asking myself if this would be enough? What are you using?

2L

Depends on state of tune. 1L should be plenty.

It depends on lots of things including how vigarously you'll be driving. There's no reason why you can't use the 0.5L one if you've got it and upgrade if you run into problems (which you probably won't unless you're regularly tracking it).