For anyone who has a set of extension tubes, here's a cheap and easy way to protect them in your camera bag. Just slide the extension tubes into a neoprene stubby/can holder.

extension tubes securely protected inside a stubby holder

The thick neoprene provides a very thick and robust holder, protecting your extension tubes from getting scratched in your camera bag.
A stubby holder is also significantly cheaper than a lens case designed specifically for photographic gear, particularly if you can get the stubby holders for nothing, as many companies give stubby holders away as promotional items, or alternatively, they can be purchased cheaply at many retail outlets as souvenir items.