Well I found this tutorial for making one that might work? It's american but I imagine you can get the same bits here: http://thepaintedhive.net/2017/02/diy-mini-barn-doors-a-complete-tutorial/ Or create something similar to this pic?

I like black (we've planned black) but I think it only works with a strong colour...the overall vibe needs to be modern.  Sage green is quite pale isn't it? More of a gentle modern country vibe perhaps? I would imagine the nickel would be much more suited in this instance.

Your kitchen plans sound absolutely stunning! For a warm neutral that complements your deep forest green cabinetry without leaning too pink or green, you might love Hardwick White or Charleston Gray from Farrow & Ball. Hardwick White has a beautifully soft, warm stone quality that works well in varying light, while Charleston Gray brings a refined taupe tone that pairs effortlessly with oak and antique brass accents. Both should add a timeless, elegant contrast while keeping the space warm and inviting. Hope that helps, and best of luck with your kitchen—it's going to look amazing!

I am assuming you mean back to front, yes I would centre them and also divide the island length into 3. That will give you the exact spot for the pendants. Well that would make it look balanced but the light also has to work so if you are happy with the positions lightwise then keep it the same. You can always have a longer cord and a cuphook to place the light whereever you need it. This is usually much more cost efficient than moving the fitting position too.