Perhaps you could spell out "H E L P ! " on the flip-side with black tape.

A few months ago I was at REI and they had a little kit on sale for $5 that included a large blaze orange "HELP" sign (would cover a good bit of my car roof) and I bought a couple. Some blaze orange items are a good idea anyway and I figured the HELP signs were worth the minimal space -- especially in winter where a blizzard could cover your car up.

Unfortunately, I can't remember the brand and tossed the packaging. Haven't been able to find it online to post the link.