B.,
I have used Cherrybrook Kitchen cakes and frostings in the past. I really like the company's prepared frosting in a plastic jar, but it is expensive. Here is the website where you can take a look at the ingredients: http://www.cherrybrookkitchen.com/
These products can be found at Meijer in the gluten free section of the grocery department. They also have a dry mix which you can add any type of liquid "milk" to make the frosting. This might be a better solution if you are making a sheet cake and need a lot of frosting.
I have also learned a few tricks because my son has 7 food intolerances including eggs and dairy. First, the icing you have been making is not a frosting, it is designed to ooze. However, to turn it into a frosting you need some type of thickener to keep it in place. The two I prefer to use need to be heated to activate: cream of tartar and tapioca starch. If you would like to continue to use the confectioners icing recipe it should be heated on a stove with a 1/2 to 1 teaspoon of thickener. The frosting will thicken very quickly so add it near the end of the heating process.
Another recipe I have used in the past, but had to thicken, I found in my husband's grandmother's Joy of Cooking cookbook from the 1970's. I have it listed below:
Banana Icing
1/2 cup banana pulp
2 cups confectioners sugar
1/8 tsp salt
1 tsp lemon juice
1/2 tsp vanilla
Mash the banana in a large mixing bowl and set aside. Sift the sugar into the mixing bowl and stir into the pulp until smooth. Beat in the salt, lemon juice, and vanilla. Add more confectioner's sugar, if needed.
You will want to try this recipe a few days before to see if any adjustments need to be made. It will not be the pretty pure white we are accustomed to, but if you are adding food coloring it shouldn't matter.
I would also try adjusting this recipe by heating adding 1/4 cup water and 1/4 tsp cream of tartar. Then heating the mixture on the stove to activate the cream of tartar and thicken the icing.
Good luck and let us know how it goes!
-C..