In one embodiment, a contiguous, elastic cover, for a dissolution-testing vessel, has a slit from a central opening to its outer edge. The cover may be placed around a stirring shaft by deforming the elastic cover to temporarily enlarge the slit. After returning to its original shape, the cover is placed over the vessel with the shaft free to rotate unimpeded within the central opening. The cover has a first raised annular disk on one side and a second raised annular disk on the other side, where the disks have different outer diameters to enable the cover to be used with differently sized vessels. The cover has a first opening having a first diameter and a second opening having a different, second diameter such that a conventional probe fits either at one depth within the first opening or at a different, second depth within the second opening.