We have both the stokers frame bag sold by Tandems East here and one made by Abus, which is designed to hold a cable lock, see the upper picture here. They both go between the rear top tube and captain's seat tube. The former is great when on tour - it takes all of our toiletries comfortably - but is a bit large for stashing a few small things for everyday riding, so we mostly use the Abus bag. They both come with us when touring, but then the Abus moves down to be between the rear lateral tube and captain's seat tube.