A rectum phantom is needed for the experiments for microwave sensor designed to sense lower abdomen. However, the rectum wall is thin which is challenging to be fabricated using the available recipes with desired electrical properties at the working frequency. In this paper, we propose a fabrication method for such a phantom using additive printed mold. The phantom has a thickness of 3 mm, which is close to that of rectal wall, and is hollow inside the wall allowing various fillers to perform experiments. For the fabrication, a multi-layered 3D printed polylactic acid (PLA) mold was designed to have the inter-layer space forming the shape of a rectum with 3-mm-thick wall. It is designed to ease demolding. Solution was poured into the space between layers, solidified, and demolded. To ensure reproductivity, the phantom’s fabrication protocol is optimized. A patch antenna connected to a vector network analyzer (VNA) is used to preliminarily measure the return loss under different filling levels of the phantom.