A sit box for use on a reformer exercise apparatus carriage is a unitary body having a rectangular flat top surface portion joining four curved portions. Each curved portion merges with one of an upright front wall, an upright back wall, an upright left side wall, and an upright right side wall. The walls terminate at a generally flat rectangular bottom parallel to the top surface portion. Each wall curves to merge with an adjacent wall. Each of the left and right side walls has an outwardly open vertical recess therein adjacent the front wall to receive within each recess one of a pair of shoulder stops projecting from the carriage.