Niftech made some like that...
www.niftech.com... they list alot of brushes, cavity as well, but no pix to be sure what they look like. But i remember seeing in a Niftech catalog, pictures of brushes that had a square cavity in them... and took up most of the face of the brush. Its been a while... but im pretty sure it was them. You can e-mail Niftech for any more info once you are on the site.
- Dave