I have nothing to do with sniping; but I would say if you are going to be somewhere warmish; take a British type NBC suit and rip out the charcoal. You will want to put it through the spin dryer a few times etc to beat it up and fade the colour; and sew in extra pockets and scrim netting as you like....
If you ae going somewhere cold, do a smiliar thing, but to a German army sleeping bag, the type that can double as a jacket.
DO NOT GET A NOMEX FLIGHT SUIT!! Mel, no offense but I was in the Navy and wore flight suits constantly. The chemical coating that makes the flight suits fire retardant does not allow the fabric to breathe at all. They are hot in the summer and cold in the winter.
I did make a ghillie suit out of an old flight suit because of the one piece design and I knew all the jute, burlap, and vegetation would nullify any temperature properties but other than that, flight suits are not the best thing to wear. They are thin and tear easily.
If you are looking for a one piece design, the better choice is to get a set of Dickies overalls. They are cheap, 100% cotton, and come in brown, khaki, OD, and black.
Just my $.02 but I think you'll make a mistake with a Nomex flight suit.
Interesting. The Nomex I had to wear the one time I gunned on a M2A2 was much better for breathability than standard BDU's. Wonder if they are the same as the ones you used? I have never built a ghillie out of one because they never seemed to be durable enough. The one "single" piece suit I built was done with mechanics coveralls (as you mentioned). I much prefer two piece ghillies and do most all of mine out of BDU's.