The water filled plant protectors will probably work like a charm. The only drawback is the small size. You stated your plants are small and these would fit. I have a similar working home made device that works. I take a five gallon plastic bucket (availiable at all the big box stores), cut the bottom out of it and make a cut from top to bottom on the side. This allows you to flex the bucket enough to get it around your plant. Put the bucket around the plant, fill with sand enough to cover the graft union for grafted plants. Throw something over the top and you are ready for a cold night. The bucket can be left on for the duration of the deleterious cold weather. Just take your cover off the top during good days. If you leave it on for extended times be sure your trunk does not sun scald when you take it off. You can paint trunks with water based latex paint that is left over from the last time you painted.