They also make
fabric straps that attach around your ankle with velcro for <$10.
If that's too expensive, you can use a rubber band, although a rubber band won't have the reflectiveness of a strap. Or, if you don't have a rubber band, just tuck your pant leg into your sock.
PS. Have you already figured out that you can tuck the shoelaces in your right foot into your shoe?
It generally doesn't work well to use a chainguard on a multi-chainring crankset. A chainguard will interfere with shifting and stick out into the pedal arm's path