DocRid - i carry small items in a sissy bar bag mounted like a saddle bag and also a fork bag - both made by La-Rosa - see post 10,Seat Time,Scottish Tour.
Since you've eliminated most of the things out there, but haven't specified how weather-proof it needs to be... a simple bungie net (usually pretty cheap, 6 hooks for the CB's luggage mounting points around the rear seat) is what I'd recommend. I have one in addition to the ROK straps and I keep both in my top-case, with that combination there's approximately nothing I can't carry on the bike.