Anne, do you get much rain or snow and ice during the winter? You might call around to appliance stores and ask if they ever have any large corrugated boxes, like those that refrigerators come in. Maybe they would save a few for you and you could place them over the top of the plants after mulching, for further protection. Only problem would be if you got lots of rain, they might get soggy and I don't know if they'd be strong enough to hold up to the weight of lots of snow, which could cause the plants to be crushed.
