If it's not to much of a fiddle, could you not keep all your stuff in a bag, in the rack pack and leave the rack pack on the bike and just take the bag with you
Just made me think..One thing i kept from my motorbike was a rack net, like a net of bungee ropes..
Cost a couple of quid, with a stuff bag it would be good I imagine. And the net will always compress it as small as the contents... No zips no Velcro and no £50 gone...