Hi Beth,
There is a product on the market called "Creative Paperclay" (and a web site with the same name) which is a white malleable putty-type substance used by dollmakers. It air dries, barely shrinks, is odorless, non toxic (good product!) and you could use it to fatten out your brush handles to your heart's content. Not only that but you can keep adding layers forever, it seems. When you have what you need, and it is dry, you can give it a coat of acrylic paint or varnish and there you have customized, hopefully painfree, brushes. Happy painting!
