When clamping the ribs at the lower block there is a tendency for the counter form to push the ribs apart, turning what should be a close fitting joint into an embarrassing gap. This is especially true if the counter-form has a smaller radius than the end block.
The counter form shown here gets around the joint opening problem, made from a cut-off from the end block, the radius is shallower than that of the finished block, the back of the counter form has been kerfed to allow flexibility.
Two layers of thick card lined with freezer paper go against the rib to take up small gaps and prevent bruising the ribs. First clamp in the center on top of the joint, locking the rib ends in place, then add the outer clamps. Ideally the finished seam will be a close fit with no glue line visible