Row cover.

Dan

Could I "make" some row cover out of contractor trash bags?

Plastic really isn't the best thermal cover -- it doesn't really insulate. If you have to use plastic, be sure to use some kind of framework to keep it up off the plants and create a dead air space for insulation. Plastic directly on plants during a cold snap actually cause more damage, IMO, as frost and ice form right on the plastic and burn the plants through it.

Nothing on your list should be harmed by snow or temperatures that fall into the mid 20's. Snow actually insulates. I wouldn't worry too much, should be OK -- check your local NWS site for a specific forecast of the lows you might get in the next few nights and see what kind of measures you might need to take.

I wouldn't worry about covering them if it won't be too cold. Temperatures in the upper twenties shouldn't harm them. Also, if you use a row cover, I would be more concerned with supporting the snow and not crushing the seedlings. If the snow accumulates, it will insulate the seedlings and protect them from the cold.

Snow is a good blanket for those plants.
Lettuce, arugula ... can even over winter in snow and cold.
I talking from personal experience. Last year my fall planted lettuces overwintered. But our coldest temms were around 14F, for many nights.