i used a stretch lace that um... had been a pseudo-tablecloth at the roaring 20's party and may have a bit of schmutz on it. whatever. it'll wash out. i kind of like the look of the interfacing as a double layer, but its weirdly flappy in the back. it may need a quick tack in the middle, or just a lower back tank top underneath
though not specifically stated in the directions, i did end up doing a double seam on the cap sleeves and a triple at the bottom hem. also, the directions suggest you sew one side, then the hem, then the remaining side. the way i cut knits can only lead to tears [that's crying not rips] and i wisely did both sides and then the bottom. only slightly uneven, but nothing a 1" hem cant fix.