i tried something similar on my first attempt except i did a clear then a black fill in a single layer. this is what i got..Here is a rough idea. It's late and I don't have time time to really refine the edges, but here is a rough idea of what I'm talking about.
Selected out the flag, then I made a layer out of it. Next I made a solid black new layer and I placed the flag over it. Like I said If I had the time I could have done a much better job of selecting.
Nice shots. I have the same setup. The D750 and the Tamron 150-600.
it's a pretty nice set up IMO...I am thinking of picking up the v2 of the Tammy. I was kinda wondering if I'd miss out on the D500 AF but that's not the case for what I do so I love having the D750 and all the FF goodness it offers.