An assembly is provided for forming a trench of a predetermined shape. The trench-forming assembly includes a removable longitudinal form body, a pair of frame members and at least one mounting bracket assembly. The form body is capable of shaping a moldable trench forming composition poured around the form. Each frame member includes a support surface, and is in engagement with a respective side surface of the form body. The mounting bracket assemblies, each of which can include a mounting bracket and a pair of hook members, extend laterally across the top surface of the form body. In this regard, each mounting bracket assembly removably engages a downwardly facing side of the support surface of the frame members.