If you look at ANH, Ben's undershirt looks to be a light cotton, almost T-shirt material. I think I can do better with some of these microweave fibers out there...
That's very true, the shirt supplied by SS has a textured pattern that is more like a sweater than the actual t-shirt-like shirt Ben wore in the film. If you do find a suitable one let us all know. It would definitely be worth the upgrade. Some of the brown t-shirts that come with Dragon soldiers might work as the collars are often more mock-neck-like than actual t-shirt necks and the material is closer to a smooth cotton.
BTW, love the paint work you did on Ben. I want to see your Leia now. Maybe you can post a pic of the two together.