From the 50s thru the 1980 in the US this method was used as well as with wheel trenchers to excavate ditches. The wheel trencher pulled a trapezoidal form behind instead of a crumbed bar that cut the slope. Once you graded the ditch line you used various systems to stay on grade while cutting ditch.  Once the ditch was dug a form very similar to ''tis was dropped in the front of the form was larger than the the rear by the thickness of the concrete used to line the ditch. The center of the form was open and concrete was fed into the opening. Vibrators on the back of the form consolidated the concrete and the form was pulled ahead. Our largest wheel trencher, a Garwood Super G, could cut a 6' bottom ditch. For the larger ditches a dozer with a winch and a short boom that helped to center the pull with the center of the ditch moved the slip form. Every 10-15' the finishers used a metal form to put in an expansion joint. The larger slip form could be configured for different trench bottoms by adding or removing spacers from the center of the slip form. The smaller slip form had a winch, like this one, that pulled against an anchor to move it forward. You should consider building your forms in an expandable configuration. You add a bolt together arrangement with a taller side to accommodate wider ditches. Build your form top as adjustable as well and sell expansion kits in sizes your customers may commonly need. Your main ditch has a 4' bottom and your branch ditches are 2' bottom so you remove your spacer to do so. Pipelines, wells, center pivots, wheel lines and lack of capital ended the hay days of these water projects in the western US. Good luck!