I've a pair of <rummages in saddlebag> Pacific Outdoor Equipment reflective velcro straps that I picked up on sale at my LBS years ago. They're fairly heavy-duty tarp-like material with a printed-on reflective layer, which is pretty effective; they work both as trouser straps and as ankle bands, but might be a bit sweaty in extended use. I don't wear them next to skin, but over my socks.
Is this for your ultra race? Depending on how the letter of the law is interpreted, might you be ok to wear socks with reflective yarn? Eg. the Rapha brevet socks have reflective and hi-viz bands woven in, and are nice and comfy.