The upper surface of the test stand must be flush with the floor of the tunnel, and the hinged struts should pivot easily and balance just aft of vertical. All other connections must be rigid; minute movements of the stand must be applied to the force beam rather than absorbed in any joints.
Using scrap ¼" plywood, cut two 2"-square vertical struts and a 2¾"×6½" test section floor. Cut the back plate 2"×2½" tall, and lighten it with a large cutout, because weight not centered in the pivoting mechanism will tend to fall backward and inflate very small measurements. Glue a ½"×¾" block to the lower-center-back of the plate; this spacer is what presses against the force beam.
Mount the underside of the test section floor to the struts, using two 1½" hinges. Glue the back plate to the aft end, and brace the connection diagonally with stiff steel wire on either side. I looped one end of each wire, screwed it next to the front strut, and bent the other end up to fit into a hole drilled in the bottom end of the back plate.
Use 2 more hinges to mount the struts to the base board, making sure the stand pivots smoothly. A reliable mechanism requires tight tolerances! I needed a snub Phillips screwdriver to handle these lower hinges.