I didn't have much luck using transparent vinyl plastic coverings over frames. I think the greenhouse effect during sunny days caused them to overheat and remain out of dormancy, so their cold resistance was not very high during the colder nights.
And I even put 3 gallon water containers inside the enclosure to attempt to help regulate temperatures, and prevent freezing. (Water releases heat as it freezes, resisting the temperature from going down much further)
I made some actual temperature measurements and, while the enclosure certainly made things much warmer inside during the day, the temperature differential between inside and outside was only 2 degrees (F) at night, and night is when temperatures are coldest. (I'm sure it may have helped resist the wind though, because of the wind chill effect, wind carrying off heat from the plant faster than still air)
This was in Olympia, WA, zone 8a.
I am guessing jute bag or frost cloth coverings might be better, since they can still provide some thermal insulation while allowing the plant to "breathe" (air can pass through) so the plant will not overheat in the sun during the daytime, so it will remain in dormancy.