The formers need to be mounted and aligned on a stable platform called a strongback. This can be made of rough timber, provided it is rigid. I had some old 25 X 230 mm shelving, some 50 X 150 mm and 50 X 50 mm pine, and from this I made up two matching deep beams, one of which is shown below. I drilled holes along the bottom edge, and inserted old bath leg adjustment screws so that I could level the final structure. I notched cross beams to ensure that the sides will be parallel, and finally simultaneously marked the former positions onto the top edge of the deep beam. I assembled the strongback and levelled it using a Bosch laser level. The result is shown below.