There's a company named Zing that produces nice neoprene covers that slip over your dSLR camera - it provides a decent amount of protection and allows you to toss your camera into your rollaway or a daypack without feeling too nervous. They're a bit overpriced, but convenient. Also, they've got a kind of goofy clip that attaches to the amera strap to keep the cover from getting lost when off the camera...just a thought.

Also, an alternative to the sigma 70-300 is the all metal constructed Nikon 75-300 - you can get it used for about the same price. It's a solid piece of work, but a bit heavy.