Lettuce germinates well with soil temps between 40°-60°. I direct seed and cover with slotted row cover as soon as possible. Your plastic cover should be fine. If there's going to be a heavy frost you can gently cover the patch with a few sheets or a light weight blanket. The frame for the plastic should be able to hold up a little extra weight over night.

Thanks again, Dear Robin! I made a beeeautiful bed of soil for this new lettuce patch- lotsa compost, manure, peat, leaf mold, alfalfa cubes, and a nice molasses dessert! I've a good heirloom lettuce seed collection and am eager to get fresh salad back in our lives! Today, whilst prepping the pea beds, i noticed that my wintered-over spinach is just beginning to grow again. Yeehaw!
Toni