In my book, painted glass is anything but standard in terms of packing solutions. For one thing the best conservator around is not going to be able to do much if it is damaged. Often basically a total loss. Just in terms of physics a huge consideration is just scale. A 4" x 6" x 1/8" piece of glass is not to going to be terrribly vulnerable if it is secured well within a frame or mount. Extra attention to cushioning components should probabably suffice in terms of shock and vibration issues. To everyone - please utilize published cushioning curves whenever possible - don't fake it! Anyway, other than being conscious of avoiding contact with pointed surfaces themselves for large glass panes no amount of cushioning is a substitute for stabilizaton. An inner container that provides rigid evenly distributed support for the entire surface of the artwork actually provides a more critical function for a substrate thats most vulnerable trait is brittleness than any amount of cushioning material. Most critically to start out with what are the sizes of the pieces in question?