Well the Flash does keep his costume in his special Flash ring, no reason he couldn't keep something in some Flash Symbol Die Cut Cuff Links like these.  Mmaybe a sandwich and some juice? All that fast running and vibrating through stuff requires carbs, hydration, and sleek, tight, cuffs. Each of these highly polished stainless steel cuff links measure approximately .875 inches ( a bit more where the bolt sticks out ) for flashy good looks.