I can't recommend this highly enough after buying one a few weeks ago:
https://www.podsacs.com/i/q/CCPA50/pod-alpine-50-back-packNormal price is £145 but they're doing it for £50 less a penny at the moment (worth paying £3 for the waterproof cover to get the free postage for > £50 orders...)
I've managed to pack the entire contents of my 80l duffel bag into it, so it will take a LOT more than 50l, although it ends up looking like you're carrying a vertical sausage on your back
Best of all, although it weighs just over 1.5kg, you can strip it back to under 1kg for lighter loads by removing the frame and the waist straps, both of which are quite substantial. It also strips down well for climbing with shock cords being removable. It takes crampons and ice axe well on the outside, too.
I have a fairly small back (I'm a 5'8" bloke) and a size A does me perfectly.