I have the AlpineStars Tech Bag, and I must say that it works quite well... The helmet is secured by 5 points on the backpack plus the bottom. It's kinda like a webbin that streches... Mind you, attaching the helmet does limit the amount of stuff you can use in the bag... I usually carry a sweater and a visor, and that was a pretty tight fit.... Other from that, it's a great bag... Has lots of reflective piping and very tuff...
4Play - If you really want to splurge, get the Dainese Multifunctional Pack... It's got the helmet holder, rain fly, and back protector all in one....