I absolutely do not reccomend detatching the lens. There is way more risk. I do have a front facing baby pouch that I fill with blankets. And then I put my jacket around the pouch.

Online I have found hard cases for certain brands. Check out b&h

Good question, I’m also concerned about taking my K10D on my coming winters holidays, I already got a bag, (not a hard case, soft one), but I think you should carry your dslr at your breast level, because personally, I do not fall on my breast while snowboarding, or at least it’s easier to prevent the fall with your arm in front of you than falling on your back.

And I also think you should let the lens attached, even if this one is a 300mm.