Hi,
Do you have somewhere secure to leave your bike at work ? If not then one alternative to a removable rack might be the Stratos range of backpacks from Osprey(see
www.osprey.com for stockists) which claim to reduce perspiration by having an open mesh layer between the bag and your back.
However if theft of removable items from your bike while you are at work isn't a problem (I'm lucky that my employer provides a secure cycle shed) then I put my lunch in 2 plastic bags which I can remove in about one second from this brilliant bag:
http://www.carradice.co.uk/sqr-saddl...ddlepack.shtml