A ridding bud of ours always wore a back pack on our up trips. we never knew why or what was in it. before we left in the morning we put about 10 beers in it. during the day he was complaining that his back was hurting, we asked for our beer in his pack, he had no clue he was carying them all day. We had a good laugh on him.
Polski first place parts has the motorfist cornice back on sale for about 60% off. There is a YouTube video showing everything a guy can put in it. Just search for it in you tube.
I wore a pack for years and if going back to the mountains to ride it is a must because you pack needs to be with you...
I now use a tunnel bag and I feel a lot better at the end of the day....Just my 2
I picked up HMK pack this year and it fit perfect and has plenty of room in it. Doesn't have extra pockets though more a an all in one pack. I carried extra gloves, a camera, extra balaclava, a hat, a map....all sorts of things and it was nice having the spare room to store some stuff.
My advice, don't get a backpack. Why support that weight with you shoulders and back when you can just put it on your sled. Better range of movement and less fatigue at the end of the day. Been down that road....
Riding buds have Klim hydration backpacks, they claim with the crossover straps to be comfortable. I did not like the one time I wore one.
If you want one check on Ebay, good prices on new from online stores.