How about something like Remay that would offer some protection without cooking them in the daytime?
Dick

Favas don't really 'thrive' in cold weather. They sit it out but they won't be growing. They survive better when they are short than taller ime. I agree that fleece would be better than plastic as it breathes and lets in moisture.

I actually ended up using a bale of hay my wife had been using for fall decoration. I spread it over the whole bed at least 6 inches deep, so even though they won't get much sun, it's only for 4-5 days and they should get some insulation from the freezing temps without getting baked during the day. I'll post about how they end up doing once we get back.