Neat idea, but usually soaking and a brush is enough. For plastic carboys I'll just shove a wet towel into the carboy and swirl it around. Works like a charm assuming the crud is softened enough from soaking.
I usually add a drop of bleach to some cold water, let it sit and the rinse thoroughly with hot water! Easy. Good point too about making your own. That'd be pretty easy and you could beef it up, bristles etc!